Production of YP170 Vitellogenins Promotes Intestinal Senescence in Caenorhabditis elegans

Abstract

During aging, etiologies of senescence cause multiple pathologies, leading to morbidity and death. To understand aging requires identification of these etiologies. For example, Caenorhabditis elegans hermaphrodites consume their own intestinal biomass to support yolk production, which in later life drives intestinal atrophy and ectopic yolk deposition. Yolk proteins (YPs; vitellogenins) exist as three abundant species: YP170, derived from vit-1-vit-5; and YP115 and YP88, derived from vit-6. Here, we show that inhibiting YP170 synthesis leads to a reciprocal increase in YP115/YP88 levels and vice versa, an effect involving posttranscriptional mechanisms. Inhibiting YP170 production alone, despite increasing YP115/YP88 synthesis, reduces intestinal atrophy as much as inhibition of all YP synthesis, which increases life span. By contrast, inhibiting YP115/YP88 production alone accelerates intestinal atrophy and reduces life span, an effect that is dependent on increased YP170 production. Thus, despite copious abundance of both YP170 and YP115/YP88, only YP170 production is coupled to intestinal atrophy and shortened life span. In addition, increasing levels of YP115/YP88 but not of YP170 increases resistance to oxidative stress; thus, longevity resulting from reduced vitellogenin synthesis is not attributable to oxidative stress resistance.

Keywords: Caenorhabditis elegans; Age-related pathology; Aging; Animal model; Yolk.

Figure 1.

Yolk accumulation results from continued vitellogenesis and cessation of egg laying. (A) Cartoon showing faucet and sink model. (B) Age changes in vitellogenin accumulation (left) and accumulation rate (right), summed data from four trials. Top, YP170; bottom, YP115. Data shown with and without adjustment for age changes in intestinal volume. (C) Combined figure showing yolk protein (YP) adjusted accumulation and accumulation rate, and other age changes. Left, with YP170; right, with YP115. (D) Mating of N2 hermaphrodites with males (fog-2) reduces YP170 accumulation. Left, total accumulation. Right, accumulation rate. (E) YP170 accumulation in spermless fog-2(q71) females. Left, total accumulation. Right, accumulation rate. (D and E) Summed data from three trials, age-matched comparison. Data are mean ± SEM, *p < .05, **p < .01, ***p < .001.

Figure 2.

Knockdown of YP170 synthesis reciprocally increases YP115/YP88 levels and vice versa. (A) Coomassie gel showing the effects of vit-5 and/or vit-6 RNAi on yolk protein (YP) levels. (B) Quantified data, fold change in YP levels. Note that vit-5 RNAi increases levels of YP115 and YP88, whereas vit-6 RNAi increases YP170 accumulation. Statistical comparisons are to L4440 control on the same day. (C) Effect of vit-5 and/or vit-6 RNAi on vit-2, vit-5 or vit-6 mRNA levels, compared to L4440 control on the same day. Note that vit-5 RNAi does not increase vit-6 mRNA levels and vit-6 RNAi does not increase vit-5 mRNA levels. (D) No change in the YP content per worm after vit-5 or vit-6 RNAi. (E) No effect of vit RNAi on total protein content. (F) Appearance of new protein bands after vit-5,-6 RNAi. All data are mean ± SEM, *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 3.

Vitellogenin synthesis reduces levels of other intestinal proteins. (A) Effects of vit RNAi on selected fluorescent intestinal reporters, compared to L4440 control on same day, data are mean ± SEM, *p < 0.05, **p < 0.01, ***p < 0.001. (B) Selected examples of effects of vit-5,-6 RNAi on intestinal reporter gene expression. Scale bars, 100 μm. (C) Effects of vit RNAi on mRNA for selected intestinal genes, compared to L4440 control on the same day; data are mean ± SEM.

Figure 4.

YP170 production accelerates intestinal atrophy and shortens life span. (A) Effect of vit RNAi on intestinal atrophy. Left, microscope images, scale bars, 10 μm. Right, quantitation. (B) Effect of vit RNAi on life span. Bar graphs depicting mean ± SEM of life spans for individual trials; p, log-rank test. (C) Effect of vit RNAi on yolk pool accumulation. Left, microscope images, scale bars, 10 μm. Right, quantitation (A and C). Filled arrowheads, intestine. Open arrowheads, PLPs. Data are mean ± SEM, age-matched comparison, *p < 0.05, ***p < 0.001.

Figure 5.

Evidence that increased YP115/YP88 can enhance oxidative stress resistance. (A and B) Effect of vit RNAi on resistance to oxidative stress caused by 40 mM paraquat (A) and 7.5 mM t-BOOH (B); bar graphs depicting mean survival ± SEM for individual trials; p, log-rank test. (C) Model for distinct roles of YP170 and YP115/YP88 in C. elegans senescence. Rate of intestinal atrophy is coupled to synthesis of YP170 but not YP115/YP88, through gut-to-yolk biomass conversion (10). Higher YP170 levels reduce life span because production is coupled to intestinal atrophy; but effects of increased YP170 cannot be ruled out. Blocking both YP170 and YP115/YP88 synthesis increases life span more than YP170 alone, suggesting that YP115/YP88 accumulation could contribute to late-life mortality. Wild-type insulin/IGF-1 signaling greatly shortens life span (35,36), increases vit gene transcription (31,37) and translation (21), and global translation (38,39), and intestinal atrophy (10).