Lysophosphatidylcholine acyltransferase 1 controls mitochondrial reactive oxygen species generation and survival of retinal photoreceptor cells
2022; Elsevier BV; Volume: 298; Issue: 6 Linguagem: Inglês
10.1016/j.jbc.2022.101958
ISSN1083-351X
AutoresKatsuyuki Nagata, Daisuke Hishikawa, Hiroshi Sagara, Masamichi Saito, Sumiko Watanabe, Takao Shimizu, Hideo Shindou,
Tópico(s)Antioxidant Activity and Oxidative Stress
ResumoDue to their high energy demands and characteristic morphology, retinal photoreceptor cells require a specialized lipid metabolism for survival and function. Accordingly, dysregulation of lipid metabolism leads to the photoreceptor cell death and retinal degeneration. Mice bearing a frameshift mutation in the gene encoding lysophosphatidylcholine acyltransferase 1 (Lpcat1), which produces saturated phosphatidylcholine (PC) composed of two saturated fatty acids, has been reported to cause spontaneous retinal degeneration in mice; however, the mechanism by which this mutation affects degeneration is unclear. In this study, we performed a detailed characterization of LPCAT1 in the retina and found that genetic deletion of Lpcat1 induces light-independent and photoreceptor-specific apoptosis in mice. Lipidomic analyses of the retina and isolated photoreceptor outer segment (OS) suggested that loss of Lpcat1 not only decreased saturated PC production but also affected membrane lipid composition, presumably by altering saturated fatty acyl-CoA availability. Furthermore, we demonstrated that Lpcat1 deletion led to increased mitochondrial reactive oxygen species levels in photoreceptor cells, but not in other retinal cells, and did not affect the OS structure or trafficking of OS-localized proteins. These results suggest that the LPCAT1-dependent production of saturated PC plays critical roles in photoreceptor maturation. Our findings highlight the therapeutic potential of saturated fatty acid metabolism in photoreceptor cell degeneration–related retinal diseases. Due to their high energy demands and characteristic morphology, retinal photoreceptor cells require a specialized lipid metabolism for survival and function. Accordingly, dysregulation of lipid metabolism leads to the photoreceptor cell death and retinal degeneration. Mice bearing a frameshift mutation in the gene encoding lysophosphatidylcholine acyltransferase 1 (Lpcat1), which produces saturated phosphatidylcholine (PC) composed of two saturated fatty acids, has been reported to cause spontaneous retinal degeneration in mice; however, the mechanism by which this mutation affects degeneration is unclear. In this study, we performed a detailed characterization of LPCAT1 in the retina and found that genetic deletion of Lpcat1 induces light-independent and photoreceptor-specific apoptosis in mice. Lipidomic analyses of the retina and isolated photoreceptor outer segment (OS) suggested that loss of Lpcat1 not only decreased saturated PC production but also affected membrane lipid composition, presumably by altering saturated fatty acyl-CoA availability. Furthermore, we demonstrated that Lpcat1 deletion led to increased mitochondrial reactive oxygen species levels in photoreceptor cells, but not in other retinal cells, and did not affect the OS structure or trafficking of OS-localized proteins. These results suggest that the LPCAT1-dependent production of saturated PC plays critical roles in photoreceptor maturation. Our findings highlight the therapeutic potential of saturated fatty acid metabolism in photoreceptor cell degeneration–related retinal diseases. In addition to the de novo pathway (Kennedy pathway) of phospholipid biosynthesis (1Kennedy E.P. Weiss S.B. The function of cytidine coenzymes in the biosynthesis of phospholipides.J. Biol. Chem. 1956; 222: 193-214Abstract Full Text PDF PubMed Google Scholar), fatty acyl moieties of membrane phospholipids are turned over dynamically by deacylation and reacylation cycles, which is called the Lands' cycle (2Lands W.E. Metabolism of glycerolipides; a comparison of lecithin and triglyceride synthesis.J. Biol. Chem. 1958; 231: 883-888Abstract Full Text PDF PubMed Google Scholar). Appropriate regulation of phospholipid composition by the Lands cycle is required for various cellular functions, including lipoprotein production and lipid mediator production (3Shimizu T. Lipid mediators in health and disease: Enzymes and receptors as therapeutic targets for the regulation of immunity and inflammation.Annu. Rev. Pharmacol. Toxicol. 2009; 49: 123-150Crossref PubMed Scopus (452) Google Scholar). In the Lands cycle, lysophospholipid acyltransferases (LPLATs) involve fatty acid (FA) reacylation to generate membrane phospholipid diversity. Depending on their substrate (lysophospholipids and acyl-CoAs) selectivity, LPLATs produce specific types of membrane phospholipids, such as polyunsaturated FA (PUFA)–containing phospholipids and saturated FA (SFA)–containing phospholipids. Among LPLATs, lysophosphatidylcholine acyltransferase 1 (LPCAT1, also known as LPLAT8) produces saturated phosphatidylcholine (PC) using lysophosphatidylcholine (LPC) and saturated fatty acyl-CoA, such as palmitoyl-CoA (4Valentine W.J. Yanagida K. Kawana H. Kono N. Noda N.N. Aoki J. Shindou H. Update and nomenclature proposal for mammalian lysophospholipid acyltransferases, which create membrane phospholipid diversity.J. Biol. Chem. 2021; 298101470PubMed Google Scholar, 5Nakanishi H. Shindou H. Hishikawa D. Harayama T. Ogasawara R. Suwabe A. Taguchi R. Shimizu T. Cloning and characterization of mouse lung-type acyl-CoA:lysophosphatidylcholine acyltransferase 1 (LPCAT1). Expression in alveolar type II cells and possible involvement in surfactant production.J. Biol. Chem. 2006; 281: 20140-20147Abstract Full Text Full Text PDF PubMed Scopus (193) Google Scholar, 6Shindou H. Shimizu T. Acyl-CoA:lysophospholipid acyltransferases.J. Biol. Chem. 2009; 284: 1-5Abstract Full Text Full Text PDF PubMed Scopus (341) Google Scholar, 7Harayama T. Eto M. Shindou H. Kita Y. Otsubo E. Hishikawa D. Ishii S. Sakimura K. Mishina M. Shimizu T. Lysophospholipid acyltransferases mediate phosphatidylcholine diversification to achieve the physical properties required in vivo.Cell Metab. 2014; 20: 295-305Abstract Full Text Full Text PDF PubMed Scopus (110) Google Scholar). Previous studies have shown that the LPCAT1-mediated production of saturated PC is required for the proper functioning of pulmonary surfactant in the lung (7Harayama T. Eto M. Shindou H. Kita Y. Otsubo E. Hishikawa D. Ishii S. Sakimura K. Mishina M. Shimizu T. Lysophospholipid acyltransferases mediate phosphatidylcholine diversification to achieve the physical properties required in vivo.Cell Metab. 2014; 20: 295-305Abstract Full Text Full Text PDF PubMed Scopus (110) Google Scholar, 8Bridges J.P. Ikegami M. Brilli L.L. Chen X. Mason R.J. Shannon J.M. LPCAT1 regulates surfactant phospholipid synthesis and is required for transitioning to air breathing in mice.J. Clin. Invest. 2010; 120: 1736-1748Crossref PubMed Scopus (108) Google Scholar) and trafficking of growth factor receptors in cancer cells (9Bi J. Ichu T.A. Zanca C. Yang H. Zhang W. Gu Y. Chowdhry S. Reed A. Ikegami S. Turner K.M. Zhang W. Villa G.R. Wu S. Quehenberger O. Yong W.H. et al.Oncogene amplification in growth factor signaling pathways renders cancers dependent on membrane lipid remodeling.Cell Metab. 2019; 30: 525-538.e8Abstract Full Text Full Text PDF PubMed Scopus (104) Google Scholar). In addition, Lpcat1 is essential for the survival of retinal photoreceptor cells (10Friedman J.S. Chang B. Krauth D.S. Lopez I. Waseem N.H. Hurd R.E. Feathers K.L. Branham K.E. Shaw M. Thomas G.E. Brooks M.J. Liu C. Bakeri H.A. Campos M.M. Maubaret C. et al.Loss of lysophosphatidylcholine acyltransferase 1 leads to photoreceptor degeneration in rd11 mice.Proc. Natl. Acad. Sci. U. S. A. 2010; 107: 15523-15528Crossref PubMed Scopus (51) Google Scholar, 11Dai X. Han J. Qi Y. Zhang H. Xiang L. Lv J. Li J. Deng W.T. Chang B. Hauswirth W.W. Pang J.J. AAV-mediated lysophosphatidylcholine acyltransferase 1 (Lpcat1) gene replacement therapy rescues retinal degeneration in rd11 mice.Invest. Ophthalmol. Vis. Sci. 2014; 55: 1724-1734Crossref PubMed Scopus (21) Google Scholar). The critical role of Lpcat1 in photoreceptor cells was originally uncovered by the analysis of retinal degeneration 11 (rd11) mice, possessing a frameshift mutation in the Lpcat1 gene. In rd11 mice, the retinal outer nuclear layer (ONL) composed of photoreceptor cells is rapidly diminished, causing vision loss until 1 month of age. Photoreceptor cells possess a unique membrane phospholipid composition (12Hamano F. Kuribayashi H. Iwagawa T. Tsuhako A. Nagata K. Sagara H. Shimizu T. Shindou H. Watanabe S. Mapping membrane lipids in the developing and adult mouse retina under physiological and pathological conditions using mass spectrometry.J. Biol. Chem. 2021; 296100303Abstract Full Text Full Text PDF PubMed Scopus (9) Google Scholar). In the membrane of photoreceptor cells, especially in the outer segment (OS) discs, docosahexaenoic acid (DHA)–containing phospholipids are extremely enriched. Our recent study revealed that the loss of DHA-containing phospholipids leads to the collapse of the OS disc structures and retinal degeneration (13Shindou H. Koso H. Sasaki J. Nakanishi H. Sagara H. Nakagawa K.M. Takahashi Y. Hishikawa D. Iizuka-Hishikawa Y. Tokumasu F. Noguchi H. Watanabe S. Sasaki T. Shimizu T. Docosahexaenoic acid preserves visual function by maintaining correct disc morphology in retinal photoreceptor cells.J. Biol. Chem. 2017; 292: 12054-12064Abstract Full Text Full Text PDF PubMed Scopus (91) Google Scholar). In parallel with this, significant levels of SFA-containing phospholipids were also present in photoreceptor cells. Therefore, the photoreceptor degeneration in rd11 suggests that the LPCAT1-mediated production of SFA-containing phospholipids contributes to photoreceptor survival. However, the molecular basis underlying retinal degeneration in rd11 mice remains unclear. In this study, we investigated the role of LPCAT1 in the retina using the Lpcat1 knockout (KO) mice. Consistent with rd11 mice, the retina of Lpcat1 KO mice showed a rapid loss of ONL with decreased levels of dipalmitoyl PC (DPPC), the major saturated PC species in the retina. We demonstrated that retinal degeneration in Lpcat1 KO mice was caused by apoptosis of photoreceptor cells, which occurs in a light-independent manner. Furthermore, Lpcat1 KO photoreceptor cells accumulated mitochondrial reactive oxygen species (ROS). Our study demonstrated that the roles of LPCAT1 are not only in DPPC production for maintenance of membrane integrity but also in the regulation of normal mitochondrial functions. A previous study has shown that a frameshift mutation in the Lpcat1 gene in rd11 and B6-JR2845 mice leads to the spontaneous development of severe retinal degeneration (10Friedman J.S. Chang B. Krauth D.S. Lopez I. Waseem N.H. Hurd R.E. Feathers K.L. Branham K.E. Shaw M. Thomas G.E. Brooks M.J. Liu C. Bakeri H.A. Campos M.M. Maubaret C. et al.Loss of lysophosphatidylcholine acyltransferase 1 leads to photoreceptor degeneration in rd11 mice.Proc. Natl. Acad. Sci. U. S. A. 2010; 107: 15523-15528Crossref PubMed Scopus (51) Google Scholar). Therefore, to investigate whether the loss of function of LPCAT1 causes retinal degeneration, we used mice genetically deleted for the Lpcat1 gene. Consistent with rd11 and B6-JR2845 mice, a dramatic reduction in ONL and OS + inner segment thickness was observed in 6-week-old Lpcat1 KO mice compared to that in Lpcat1 wildtype (WT) and heterozygous (HZ) mice (Fig. 1, A and B). Meanwhile, the thickness of the inner nuclear layer and ganglion cell layer were similar among the three groups (Fig. 1, A and B). The histological differences between Lpcat1 WT and HZ mouse retinas were not observed. Apoptosis of photoreceptor cells is a major cause of retinal degeneration (14Hamel C. Retinitis pigmentosa.Orphanet J. Rare Dis. 2006; 1: 40Crossref PubMed Scopus (663) Google Scholar). To assess the cause of retinal degeneration in Lpcat1 KO mice, we visualized the apoptotic cells in the retina using terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL). TUNEL analyses indicated that the number of apoptotic photoreceptor cells was higher in 4-week-old Lpcat1 KO mice than in HZ mice (Fig. 1C). Notably, the increase in apoptotic cells in global Lpcat1 KO mice was not observed in the lungs, liver, or other retinal cell layers (Figs. 1C and S1, A and B). These results indicate that loss of Lpcat1 leads to photoreceptor cell apoptosis in a specific manner. To assess the onset of photoreceptor cell apoptosis in Lpcat1 KO mice, we performed TUNEL staining of the retina at different time points. Apoptotic cells in the retina of Lpcat1 KO mice increased greatly from postnatal day 8 (P8) (Fig. 2A). Consistent with this, the thickness of the ONL was thinner in Lpcat1 KO mice at P8, while no difference in the apoptotic cell number and ONL thickness was seen in the P6 retina (Fig. 2, A and B). Dramatic increases in the expression of rhodopsin, critical for phototransduction and OS formation (15Lem J. Krasnoperova N.V. Calvert P.D. Kosaras B. Cameron D.A. Nicolo M. Makino C.L. Sidman R.L. Morphological, physiological, and biochemical changes in rhodopsin knockout mice.Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 736-741Crossref PubMed Scopus (341) Google Scholar), at this time point (Fig. 2C), suggests that photoreceptor apoptosis in Lpcat1 KO retinas occurred concomitantly with photoreceptor OS formation. As OS is required for phototransduction, we hypothesized that photoreceptor cell death in Lpcat1 KO mice is related to light exposure. Thus, we investigated the apoptosis of photoreceptor cells in P9 mice, which were raised in complete darkness from P2 to P9. However, photoreceptor cell apoptosis was observed even in dark-reared Lpcat1 KO mice (Fig. 2D), suggesting that retinal degeneration in Lpcat1 KO mice was independent of light stimulation. Photoreceptor cells undergo dynamic morphological and metabolic alterations during retinal maturation (16Fu Z. Kern T.S. Hellstrom A. Smith L.E.H. Fatty acid oxidation and photoreceptor metabolic needs.J. Lipid Res. 2021; 62100035Abstract Full Text Full Text PDF Google Scholar). Since LPCAT1 produces SFA-containing PC species, we next analyzed PC composition and LPCAT1 expression during retinal maturation. As shown in Figure 3, liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis showed a gradual alteration in retinal PC composition during maturation. Consistent with previous studies (12Hamano F. Kuribayashi H. Iwagawa T. Tsuhako A. Nagata K. Sagara H. Shimizu T. Shindou H. Watanabe S. Mapping membrane lipids in the developing and adult mouse retina under physiological and pathological conditions using mass spectrometry.J. Biol. Chem. 2021; 296100303Abstract Full Text Full Text PDF PubMed Scopus (9) Google Scholar, 13Shindou H. Koso H. Sasaki J. Nakanishi H. Sagara H. Nakagawa K.M. Takahashi Y. Hishikawa D. Iizuka-Hishikawa Y. Tokumasu F. Noguchi H. Watanabe S. Sasaki T. Shimizu T. Docosahexaenoic acid preserves visual function by maintaining correct disc morphology in retinal photoreceptor cells.J. Biol. Chem. 2017; 292: 12054-12064Abstract Full Text Full Text PDF PubMed Scopus (91) Google Scholar), the proportion of DHA-containing PC species, including PC 38:6, PC 40:6, and PC 44:12, increased along with retinal maturation (Fig. 3). Coinciding with the onset of photoreceptor cell apoptosis in Lpcat1 KO mice, DPPC, a major product of LPCAT1, was increased in the retina of WT mice between 1 and 2 weeks of age (Fig. 3, A and B). Consistent with the substrate selectivity of LPCAT1 in vitro (5Nakanishi H. Shindou H. Hishikawa D. Harayama T. Ogasawara R. Suwabe A. Taguchi R. Shimizu T. Cloning and characterization of mouse lung-type acyl-CoA:lysophosphatidylcholine acyltransferase 1 (LPCAT1). Expression in alveolar type II cells and possible involvement in surfactant production.J. Biol. Chem. 2006; 281: 20140-20147Abstract Full Text Full Text PDF PubMed Scopus (193) Google Scholar, 7Harayama T. Eto M. Shindou H. Kita Y. Otsubo E. Hishikawa D. Ishii S. Sakimura K. Mishina M. Shimizu T. Lysophospholipid acyltransferases mediate phosphatidylcholine diversification to achieve the physical properties required in vivo.Cell Metab. 2014; 20: 295-305Abstract Full Text Full Text PDF PubMed Scopus (110) Google Scholar), DPPC levels were decreased by half in Lpcat1 KO retinas compared to WT retinas. Thus, DPPC production in the retina largely depends on LPCAT1 expression. However, because the induction of Lpcat1 mRNA and protein was not observed during retinal maturation (Fig. S2, A and B), post-translational modifications or the increased substrate supply for LPCAT1, such as LPC and palmitoyl-CoA, may affect the age-dependent elevation of DPPC. Next, we aimed to clarify the cause of photoreceptor cell death in Lpcat1 KO mice by comparing the mRNA expression profiles with the control mice. First, we analyzed the transcriptomic differences in the 2-week-old retinas of Lpcat1 WT and KO mice (Fig. 4, A and B). Gene groups that increased significantly in Lpcat1 KO retinas were analyzed by functional classification using "the Database for Annotation, Visualization, and Integrated Discovery." These results suggest that many of the genes upregulated in Lpcat1 KO retinas were related to immune and inflammatory responses and genes downregulated in Lpcat1 KO retinas were involved in the cell projection organization and cilium movement (Fig. 4B and Table S1). In addition, we found that the expression of genes termed immediate early genes (IEGs), such as Fosb, Fos, Egr1, and Egr2, was higher in Lpcat1 KO than in the control retina (Table S1). Among them, we found that Fosb expression in Lpcat1 KO was significantly higher than in the control retinas, even in dark-reared mice (Fig. 4, C and D). As IEGs are rapidly upregulated by various cellular stimuli, particularly in neurons (17Sheng M. Greenberg M.E. The regulation and function of c-fos and other immediate early genes in the nervous system.Neuron. 1990; 4: 477-485Abstract Full Text PDF PubMed Scopus (2045) Google Scholar), we performed qPCR analyses using isolated a photoreceptor cell marker CD73-positive and CD73-negative cells (18Koso H. Minami C. Tabata Y. Inoue M. Sasaki E. Satoh S. Watanabe S. CD73, a novel cell surface antigen that characterizes retinal photoreceptor precursor cells.Invest. Ophthalmol. Vis. Sci. 2009; 50: 5411-5418Crossref PubMed Scopus (59) Google Scholar) from the retina to identify the cellular sources of their upregulation (Fig. S3A). The qPCR results showed increased expression of IEGs in CD73-negative cells (Figs. 4E and S3B). The induction of Fosb in Lpcat1 KO retinas was slightly delayed from the onset of retinal degeneration (Fig. 4C), suggesting that it might be triggered by apoptosis of photoreceptor cells. To clarify the primary cause of photoreceptor cell apoptosis in Lpcat1 KO mice, we performed microarray analysis using isolated photoreceptor cells at P8 when apoptosis of photoreceptor cells in Lpcat1 KO mice begins. As a result of functional classification analysis, no significant enrichment of the biological process altered in Lpcat1 KO photoreceptor cells was found in the gene list, except for the categories of cell adhesion (Fig. S3, C and D). Since most of the downregulated genes listed in these categories in Lpcat1 KO photoreceptor cells were reported to be highly expressed in retinal pigment epithelial (RPE) cells, it may reflect the reduced contamination of RPE cell fragments in isolated photoreceptor cells in Lpcat1 KO mice. Consistently, qPCR analysis showed that mRNA expression of RPE65, an RPE-specific gene, was lower in isolated photoreceptor cells of Lpcat1 KO mice than in control mice (Fig. S3E). Although the cause of this observation is currently unknown, transcriptomic analysis of isolated photoreceptor cells suggested that loss of Lpcat1 did not induce the obvious changes in the gene expression profiles in photoreceptor cells. A previous study reported that the failure of SFA-containing PC production due to loss of LPCAT1 leads to excessive PUFA accumulation and endoplasmic reticulum (ER) stress response in the retina (19Akagi S. Kono N. Ariyama H. Shindou H. Shimizu T. Arai H. Lysophosphatidylcholine acyltransferase 1 protects against cytotoxicity induced by polyunsaturated fatty acids.FASEB J. 2016; 30: 2027-2039Crossref PubMed Scopus (20) Google Scholar). However, under the present assay conditions, no induction of CCAAT-enhancer-binding protein homologous protein, an ER stress marker, was observed both in Lpcat1 KO retina and isolated photoreceptor cells (Fig. S3F). Based on the microarray analyses showing no obvious causative changes in photoreceptor cell death in Lpcat1 KO mice, we investigated whether the altered membrane phospholipid composition in Lpcat1 KO photoreceptor cells influences the localization and functions of proteins essential for photoreceptor survival. Consistent with the previous observation of Lpcat1 mutant mice (10Friedman J.S. Chang B. Krauth D.S. Lopez I. Waseem N.H. Hurd R.E. Feathers K.L. Branham K.E. Shaw M. Thomas G.E. Brooks M.J. Liu C. Bakeri H.A. Campos M.M. Maubaret C. et al.Loss of lysophosphatidylcholine acyltransferase 1 leads to photoreceptor degeneration in rd11 mice.Proc. Natl. Acad. Sci. U. S. A. 2010; 107: 15523-15528Crossref PubMed Scopus (51) Google Scholar), the structure of Lpcat1 KO OS discs photoreceptor cells appeared normal (Fig. 5A). We then explored the possibility that an altered PC composition of Lpcat1 KO photoreceptor OS membrane affected the localization and function of phototransduction-related proteins since the mislocalization of photoreceptor OS-localized proteins, such as rhodopsin and phosphodiesterase 6β (PDE6β), is observed in various types of retinal degeneration (20Mendes H.F. van der Spuy J. Chapple J.P. Cheetham M.E. Mechanisms of cell death in rhodopsin retinitis pigmentosa: Implications for therapy.Trends Mol. Med. 2005; 11: 177-185Abstract Full Text Full Text PDF PubMed Scopus (310) Google Scholar, 21Chang B. Hawes N.L. Pardue M.T. German A.M. Hurd R.E. Davisson M.T. Nusinowitz S. Rengarajan K. Boyd A.P. Sidney S.S. Phillips M.J. Stewart R.E. Chaudhury R. Nickerson J.M. Heckenlively J.R. et al.Two mouse retinal degenerations caused by missense mutations in the beta-subunit of rod cGMP phosphodiesterase gene.Vis. Res. 2007; 47: 624-633Crossref PubMed Scopus (290) Google Scholar). In parallel with the DHA-containing PCs enriched in the OS discs (13Shindou H. Koso H. Sasaki J. Nakanishi H. Sagara H. Nakagawa K.M. Takahashi Y. Hishikawa D. Iizuka-Hishikawa Y. Tokumasu F. Noguchi H. Watanabe S. Sasaki T. Shimizu T. Docosahexaenoic acid preserves visual function by maintaining correct disc morphology in retinal photoreceptor cells.J. Biol. Chem. 2017; 292: 12054-12064Abstract Full Text Full Text PDF PubMed Scopus (91) Google Scholar), we found that DPPC was also abundant in the OS and decreased by half in Lpcat1 KO (Fig. 5B). This result suggested that the LPCAT1-mediated production of DPPC contributes to maintaining proper PC composition in the photoreceptor OS membrane. As we found that Lpcat1 deletion leads to altered OS membrane PC composition, we next investigated the localization of rhodopsin and PDE6β in the retina. Unexpectedly, both rhodopsin and PDE6β were normally localized in the photoreceptor OS in Lpcat1 KO mice, despite the altered membrane PC composition in the OS (Fig. 5C). PDE6β is a rod photoreceptor–specific subunit of PDE that controls cyclic nucleotide-gated channel–mediated cation influx into photoreceptor cells (22Arshavsky V.Y. Lamb T.D. Pugh Jr., E.N. G proteins and phototransduction.Annu. Rev. Physiol. 2002; 64: 153-187Crossref PubMed Scopus (522) Google Scholar). Mice harboring the loss-of-function mutant of PDE6β (termed rd1 mouse) showed marked cGMP accumulation and subsequent severe retinal degeneration (23Lolley R.N. Farber D.B. Abnormal guanosine 3', 5'-monophosphate during photoreceptor degeneration in the inherited retinal disorder of C3H/HeJ mice.Ann. Ophthalmol. 1976; 8: 469-473PubMed Google Scholar, 24Chang B. Hawes N.L. Hurd R.E. Davisson M.T. Nusinowitz S. Heckenlively J.R. Retinal degeneration mutants in the mouse.Vis. 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Since the accumulated cGMP-dependent increase in calcium ion influx triggers retinal degeneration in rd1 mice, the underlying mechanisms of retinal degeneration in Lpcat1 KO mice differ from those in PDE6β mutated mice (rd1). Excess SFA induces apoptosis (25Estadella D. da Penha Oller do Nascimento C.M. Oyama L.M. Ribeiro E.B. Damaso A.R. de Piano A. Lipotoxicity: Effects of dietary saturated and transfatty acids.Mediators Inflamm. 2013; 2013137579Crossref Scopus (132) Google Scholar). Similarly, stearoyl-CoA desaturase (SCD), which converts saturated fatty acyl-CoA to monounsaturated fatty acyl-CoA, protects cells from SFA-induced cell death (26Scaglia N. Igal R.A. Stearoyl-CoA desaturase is involved in the control of proliferation, anchorage-independent growth, and survival in human transformed cells.J. Biol. Chem. 2005; 280: 25339-25349Abstract Full Text Full Text PDF PubMed Scopus (132) Google Scholar). Together with the significant decrease in the PC 32:1/PC 32:0 ratio (Fig. S4A) and SCD mRNA (Fig. S4B) during photoreceptor maturation, mature photoreceptor cells are presumably less potent in reducing the intracellular SFA levels by the unsaturation. We hypothesized that SFA stress is involved in photoreceptor-specific apoptosis in Lpcat1 KO mice based on these results. Several factors, including ceramide accumulation, mitochondrial ROS production, and ER stress, trigger SFA-induced apoptosis (27Chen L. Ren J. Yang L. Li Y. Fu J. Li Y. Tian Y. Qiu F. Liu Z. Qiu Y. Stearoyl-CoA desaturase-1 mediated cell apoptosis in colorectal cancer by promoting ceramide synthesis.Sci. Rep. 2016; 6: 19665Crossref PubMed Scopus (78) Google Scholar, 28Egnatchik R.A. Leamy A.K. Noguchi Y. Shiota M. Young J.D. 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A slight increase in sphingomyelin (SM) suggested that loss of Lpcat1 increased palmitoyl-CoA availability for sphingolipid synthesis. However, we could not find any clear differences in ceramide levels and compositions between genotypes (Figs. 4C and 6, B and C). Finally, we explored the possibility that the Lpcat1 deletion leads to increased mitochondrial FA β-oxidation and consequent ROS accumulation in mature photoreceptor cells. We analyzed the mitochondrial ROS accumulation in retinal cells of P21 control and Lpcat1 KO mice. Flow cytometric analyses of retinal CD73-positive (photoreceptor) cells showed that the ROS accumulated photoreceptor cells in Lpcat1 KO mice were increased compared to the control mice (Figs. 6D and S5, A–C). Consistent with TUNEL staining, we observed no differences in ROS accumulation between Lpcat1 KO and control CD73-negative cells (Fig. 6E). Additionally, intracellular ROS levels were also increased in Lpcat1 KO photoreceptor cells (Fig. S6, A–E). These results suggest that mitochondrial ROS accumulation triggered by Lpcat1 deletion is a photoreceptor-specific phenomenon contributing to apoptosis induction. Here, we demonstrated that the loss of Lpcat1 leads to an early onset of severe retinal degeneration, triggered by light stimulus–independent photoreceptor cell apoptosis. Although photoreceptor cell damage induces various types of retinal degeneration, the molecular basis of photoreceptor cell death is not fully understood. Our present study suggests that Lpcat1 deletion–triggered disruption of palmitoyl-CoA flux leads to mitochondrial ROS accumulation and denaturation in photoreceptor cells. FA saturation of membrane phospholipids affects membrane fluidity and flexibility through their biophysical properties (31Harayama T. Riezman H. Understanding the diversity of membrane lipid composition.Nat. Rev. Mol. Cell Biol. 2018; 19: 281-296Crossref PubMed Scopus (920) Google Scholar, 32Uematsu M. Shimizu T. Raman microscopy-based quantification of the physical properties of intracellular lipids.Commun. Biol. 2021; 4: 1176Crossref PubMed Scopus (9) Google Scholar). A recent study showed that PUFA-containing phospholipids, especially in DHA-containing phospholipids, are enriched in the center of photoreceptor discs. In contrast, SFA-containing species are enriched in the rim regio
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