UV-photoconversion of ethosuximide from a longevity-promoting compound to a potent toxin

Abstract

The anticonvulsant ethosuximide has been previously shown to increase life span and promote healthspan in the nematode Caenorhabditis elegans at millimolar concentrations. Here we report that following exposure to ultraviolet irradiation at 254 nm, ethosuximide is converted into a compound that displays toxicity toward C. elegans. This effect is specific for ethosuximide, as the structurally related compounds trimethadione and succinimide do not show similar toxicities following UV exposure. Killing by UV-irradiated ethosuximide is not attenuated in chemosensory mutants that are resistant to toxicity associated with high doses of non-irradiated ethosuximide. Non-irradiated ethosuximide extends life span at 15°C or 20°C, but not at 25°C, while irradiated ethosuximide shows similar toxicity at all three temperatures. Dietary restriction by bacterial deprivation does not protect against toxicity from irradiated ethosuximide, while non-irradiated ethosuximide further extends the long life spans of restricted animals. These data support the model that ethosuximide extends life span by a mechanism that is, at least partially, distinct from dietary restriction by bacterial deprivation and demonstrates an unexpected photochemical conversion of ethosuximide into a toxic compound by UV light.

Figure 1. Ethosuximide treatment decreases the adult life span in C. elegans when UV-irradiated with the bacterial food source and NGM.

(A) The ethosuximide (UV) treated N2 worms showed shortened life span as compared to untreated worms. (B) Wild-type and ethosuximide (UV) treated animals after 6 days from egg. Ethosuximide treated worms have severe defects and reduced viability. Several animals were selected, fixed onto a slide, and photographed. Images representative of the majority of animals were selected.

Figure 2. Duration of UV treatment of ethosuximide affects toxicity, absorption spectrum, and longevity of C. elegans.

(A) Ethosuximide was exposed to UV-light (254nm) for varying lengths of time. L4 worms were transferred to Amp/FUDR plates containing UV-treated ethosuximide added topically to a final concentration of 10 mM and were scored for life span. Increased length of UV-exposure, led to an increased toxicity, as measured by a shortened life span. (B) Ethosuximide was exposed to UV light for various lengths of time (shown in graph legend). An absorption spectrum over the ranges of 220nm to 400nm was collected for the untreated and UV-treated ethosuximide solutions. (C) Ethosuximide delays development in wild-type (N2) worms. Eggs were transferred to NGM plates with topical addition of water (control), 30 mM ethosuximide, or 30 mM UV-irradiated ethosuximide at 20°C. The developmental stage of each worm was determined after 1, 2, 4 and 6 days as L1/L2, L3 and L4 larvae and young adult/egg-laying adult nematodes. Data from two experiments (3 independent plates each experiment) were pooled.

Figure 3. Other anticonvulsants do not exhibit similar UV-mediated toxicity in C. elegans.

(A) Structures of ethosuximide, trimethadione and succinimide. (B) Unlike ethosuximide, UV-irradiated trimethadione and UV-irradiate succinimide caused no detectable toxicity. (C) Heat-treated ethosuximide also did not cause toxicity similar to UV-treated ethosuximide toxicity. L4 worms were transferred to plates containing 5, 10, 50 or 100 mM heat-treated ethosuximide, 5, 10, 50 or 100 mM UV-treated ethosuximide, or ethosuximide (control). Plates were scored for the proportion of viable worms at day 7 of adulthood. The error bars represent variation of three independent experiments (mean ± SEM).

Figure 4. Chemosensory mutants resistant to high-dose ethosuximide do not show similar resistance to UV-treated ethosuximide.

Eggs from N2, osm-3(p802), che-3(p801), and che-3(am165) worms were laid on plates containing topically added ethosuximide or UV-ethosuximide at 10, 30, 70, 80 or 85 mM, as indicated (volume matched water, control). Plates were scored for survival 5 days later.

Figure 5. Ethosuximide treatment without UV-exposure extends the adult life span in C. elegans in a temperature-dependent manner.

NGM containing 10-type (N2) animals at (A) 15°C and (B) 20°C, but not at (C) 25°C. Irradiated ethosuximide showed similar toxicity at all three temperatures. (D) Dietary restriction further extended the life span of ethosuximide-treated worms, but failed to rescue the short life span of UV-ethosuximide treated worms.