Folic acid alleviates age-related cognitive decline and inhibits apoptosis of neurocytes in senescence-accelerated mouse prone 8: deoxythymidine triphosphate biosynthesis as a potential mechanism

Abstract

Disturbed deoxythymidine triphosphate biosynthesis due to the inhibition of thymidylate synthase (TS) can lead to uracil accumulation in DNA, eventually, lead to neurocytes apoptosis and cognitive decline. Folic acid supplementation delayed cognitive decline and neurodegeneration in senescence-accelerated mouse prone 8 (SAMP8). Whether folic acid, one of nutrition factor, the effect on the expression of TS is unknown. The study aimed to determine if folic acid supplementation could alleviate age-related cognitive decline and apoptosis of neurocytes by increasing TS expression in SAMP8 mice. According to folic acid concentration in diet, four-month-old male SAMP8 mice were randomly divided into three different diet groups by baseline body weight in equal numbers. Moreover, to evaluate the role of TS, a TS inhibitor was injected intraperitoneal. Cognitive test, apoptosis rates of neurocytes, expression of TS, relative uracil level in telomere, and telomere length in brain tissue were detected. The results showed that folic acid supplementation decreased deoxyuridine monophosphate accumulation, uracil misincorporation in telomere, alleviated telomere length shorting, increased expression of TS, then decreased apoptosis rates of neurocytes, and alleviated cognitive performance in SAMP8 mice. Moreover, at the same concentration of folic acid, TS inhibitor raltitrexed increased deoxyuridine monophosphate accumulation, uracil misincorporation in telomere, and exacerbated telomere length shorting, decreased expression of TS, then increased apoptosis rates of neurocytes, and decreased cognitive performance in SAMP8 mice. In conclusion, folic acid supplementation alleviated age-related cognitive decline and inhibited apoptosis of neurocytes by increasing TS expression in SAMP8 mice.

Introduction

There are 50 million people lived with dementia in the world in 2018, a global community around the size of South Korea or Spain, among them about two-thirds was Alzheimer's disease (AD) [1]. AD is an age-related neurodegenerative disease characterized by memory loss and cognitive decline [2]. A large proportion of Chinese adults have a low folate status. Hao L et al. estimated that about 40% of the Northerners and 6% of the Southerners had plasma folate concentrations lower than the 6.8 nmol/L (3 μg/L) [3]. The United States and Canada mandated fortification of grain products with folic acid in 1998 to prevent neural tube defects. Fortification effectively increased folic acid intake and folate status of the US population, and the incidence of neural tube defects decreased substantially after fortification [4]. Folate deficiency can increase the risk of mild cognitive impairment and AD, while folic acid supplementation can improve cognitive function [5,6].

It is suggested that brain cell aging and apoptosis may be involved in the pathogenesis of AD [7,8]. Uracil may occur in DNA as a result of thymine replacing incorporation or cytosine deamination [9]. Its quantitative characterization is critical in assessing DNA damages in cells with flustered metabolism of thymidylate [10]. Thymidylate synthase (TS) catalyzes methylation of deoxyuridine monophosphate (dUMP) producing deoxythymidine monophosphate (dTMP) though de novo thymidylate biosynthesis, which 5,10-methylene tetrahydrofolic participates in this process as a one carbon (methyl)-group donor [11,12]. Abnormally elevated dUTP/dTTP ratios will result in uracil misincorporation [13], inducing a cycle of base excision repair, increased frequency of abasic sites, and enhanced risk of double-strand breaks (DSB), which increasing chromosomal instability [14].

Telomeres are nucleoprotein structures at eukaryotic chromosome ends [15], which are essential for protecting against degradation by nucleases, end-to-end fusions [16], and chromosome rearrangement. Telomeres are vulnerable to uracil misincorporation, incomplete excision repair of uracil, causing abasic sites, DSB, and compromised telosome formation due to the thymidine-rich characteristic of telomere [17]. Telomere damage is a potential initiating factor for degenerative disease and cancer due to chromosome instability [18], [19], [20]. Telomere attrition can result in potentially maladjusted cellular changes, including apoptosis [21], block cell division, disturb tissue reproduction, and finally cause cell aging and death [22]. Folate acts as a coenzyme to transfer one-carbon units that are necessary for deoxythymidylate synthesis, purine synthesis, and various methylation reactions [23]. Folate deprivation results in DNA injuries such as increased uracil misincorporation, genomic DNA strand breaks, and global DNA hypomethylation [24]. Some studies considered that serum folate levels are associated with telomere length [25,26]. However, whether folate regulates telomere length through inducing TS expression is unknown.

Our previous study found that folic acid inhibited apoptosis in astrocytes in vitro. This protective effect may be due to folic acid decreased oxidative stress, thereby preventing telomeric DNA oxidative damage and telomere attrition [27]. Moreover, folic acid supplementation delays age-related neurodegeneration and cognitive decline in SAMP8 mice, in which alleviated telomere attrition could serve as one influential factor in the process [28]. Based on our previous study, the in-depth mechanism of the effect of folic acid on telomere attrition was discussed in this study. Folate takes part in the synthesis of thymidylic acid. So in this study, we hypothesized that folic acid supplementation could reduce telomere shortening, which might be related to mediate the synthesis of thymidylic acid, reducing uracil misincorporation into telomeres. This study aimed to determine if folic acid supplementation could alleviate age-related cognitive decline and apoptosis of neurocytes by decreasing deoxyuridine monophosphate accumulation and uracil misincorporation, increasing the expression of TS, then reducing telomere shortening in SAMP8 mice.