Relating estradiol and telomeres to longevity in marine medaka Oryzias melastigma

Abstract

Longevity gender gap (LGG) (females living longer than males) exists in humans and other animals. Ample evidence in mammals has shown that a gender difference exists in telomere length (TL) (females longer than males) and the rate of telomere attrition (females slower than males). Critical shortening of telomere may induce replicative senescence and ultimately lead to tissue stem cell exhaustion, age-related tissue degeneration, organismal aging and shortened lifespan. In mammals, estradiol (E2) has been proposed as a key factor responsible for reducing telomere attrition via two pathways: (1) increase telomerase activity (TA) to replenish telomeres loss, and/or (2) reduce oxidative damage to telomeric DNA. However, the relationships between E2, TL and longevity in non-mammalian vertebrates remain largely unexplored. Marine medaka Oryzias melastigma possess a number of desirable characteristics for telomere and estrogen biology and longevity studies, including: (1) short TL (0.5-12 kb); (2) gradual senescence and telomere shortening with age; (3) known telomerase reverse transcriptase (TERT) and estrogen receptor-alpha (ERα) gene and protein sequences; (4) short lifespan (approximately 18 months), and available in large numbers under laboratory conditions; (5) estrogen biology similar to humans, and (6) distinct sexual dimorphism, the morphology of the anal fin is very prominent in O. melastigma ca. 1 month after hatching, rendering it highly desirable for gender study. In the present study, the O. melastigma was used as a model for studying the relationship between TL, E2 and longevity in non-mammalian vertebrates. The sex ratio and fish survival, plasma E2, testosterone levels of O. melastigma at 4 months (young), 8 months (middle-aged) and 12 months old (senior) were measured. Telomere length (by Southern blotting), telomerase activity (by RTQ-TRAP assay), gene expressions of telomerase reverse transcriptase (TERT) and ERα (by real time-PCR), DNA oxidation (by 8-OHdG) and protein oxidation (by protein carbonyl) were measured in the liver and gills of male and female O. melastigma at different ages. The results show that LGG also exists in O. melastigma (females living longer than males). The female O. melastigma has begun to outnumber the males by the time they reach 10 months old. With advancing age, the proportion of short TRFs length (˂2 kb) increases in the liver of female and male O. melastigma and in the gills of female O. melastigma, showing a decline of TL with age. In the liver, gender difference is clearly evident in TL, TA, TERT and ERα mRNA expressions in the young and middle-aged O. melastigma. Plasma E2 was positively correlated with liver ERα mRNA expression (p ≤ 0.001) and liver TA (p ≤ 0.05). In addition, liver ERα mRNA expression was positively correlated with liver TA (p ≤ 0.001), suggesting up-regulation of TA by E2 in the liver was possibly mediated via ERα regulated pathways: TERT gene transcription and/or TERT protein phosphorylation. In parallel, liver DNA oxidation (as measured by 8-OHdG level) and oxidative protein damage (as measured by protein carbonyl level) were found to increase with age in O. melastigma, and the levels of oxidative damage were generally lower in females than in males. In addition, plasma E2 was inversely correlated with 8-OHdG (p ≤ 0.05) and protein carbonyl content (p ≤ 0.001), respectively, suggesting E2 may enhance cellular antioxidant defense in the liver, leading to a lower oxidative damage in the females. Collectively, in the liver, E2 may prevent telomere shortening by enhancing telomerase activity and reducing oxidation damage. In the gills, gender difference in TL and TA were detected, but not for TERT mRNA expression. Given an absence of ERα mRNA expression in the gills, and a significant positive correlation between plasma E2 and gill TA (p ≤ 0.001), the activity of telomerase in the gills of O. melastigma was unlikely to be mediated via TERT transcription, but likely to be induced by E2 via the TERT protein phosphorylation pathways. Moreover, the gill 8-OHdG and protein carbonyl levels were lower in middle-aged females than in males (8 months old), possibly because females have stronger antioxidant defense capacity. When plasma E2 declined in old female O. melastigma (12 months), the antioxidant defense capacity in the female gills was weakened and the gender advantage faded out accordingly. In conclusion, the results of this study provide a comprehensive understanding of the in vivo relationship between E2, TL and longevity in O. melastigma. The present findings support the view that E2 may also protect telomeres by increasing TA and reducing cellular oxidative stress and is also a key factor contributing to a greater longevity in female O. melastigma. Given the short generation time and distinct sexual dimorphism, O. melastigma may serve as a desirable alternative model for further studies of the possible mechanisms of gender-specific longevity in vertebrates.