Aging represents a fundamental biological phenomenon; it can be considered as an inevitable progressive process related to the accumulation of oxidative damage. During normal aging, the brain undergoes morphologic and functional changes resulting in the observed behavioral retrogression such as declines in motor and cognitive performance. These declines are augmented by neurodegenerative diseases including amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), and Parkinson's disease (PD) [32
]. Currently it has been an encouraging challenge for genetic biology to discover an effective therapy for aging. Therapies in aging may be aimed at reversing or decelerating age-related modifications once they have occurred at various points in the life span [33
]. D-gal is a reducing sugar and can be metabolized at normal concentration. However, at high levels, it can be converted into aldose and hydroperoxide under the catalysis of galactose oxidase, resulting in the generation of a superoxide anion and oxygen-derived free radicals [10
]. D-gal also reacted readily with the free amines of amino acids in proteins and peptides in vivo to form advanced glycation endproducts (AGEs) [10
]. Evidence showed that AGEs could remarkably cause the accumulation of ROS and also trigger neuronal inflammatory through activation of NF-κ
B pathway, which induced the behavioral impairment of learning and memory and decreased antioxidant enzyme activities in the brain. By this token, ROS, AGEs, and inflammatory response induced by D-gal contributed to the neurotoxic effect [10
]. These changes are considerably similar to the normal aging process. So the mouse treated with D-gal has been served as the aging model and has been utilized to investigate the mechanism related to the aging. Flavonoids are unique phytochemicals and have powerful antioxidants. They appear potential in antiaging and anti-inflammation properties [35
]. Purple sweet potato color is one of the major flavonoids and has displayed some physiological effects and food exploitation. In this study, PSPC is mostly composed of cyanidin acyl glucosides and peonidin acyl glucosides (peonidin 3-O-(6-O-(E)-caffeoyl-2-O-β
-D-glucoside, peonidin 3-O-(2-O-(6-O-(E)-caffeoyl-β
-D-glucopyranoside, peonidin 3-O-(2-O-(6-O-(E)-feruloyl-β
-Dglucopyranoside, cyanidin 3-O-(6-O-p-coumaroyl)-β
-D-glucopyranoside) by HPLC analysis [37
]. Evidence showed that cyanidin acyl glucosides and peonidin acyl glucosides had higher antioxidant and anti-inflammatory capacity [38
]. The acylated cyanidin and peonidin as the major components of PSPC played an important role in pharmacological effect of PSPC. Our results also demonstrated that the PSPC is an effective antioxidant and anti-inflammatory agent. It could alleviate or reverse the D-gal-caused neurotoxicity by regulating the ROS levels in mouse brain.
In our behavioral tests, we found that D-gal could remarkably lead to the impairment of learning and memory in mice. However, both open-field and passive avoidance task suggested that there appeared a novel effect of PSPC on the improvement of brain functions during aging. On one hand, PSPC significantly inhibited the behavioral retrogression, such as rearing/leaning, grooming, and crossing, induced by D-gal. PSPC could also reverse the impairment of learning and memory in D-gal-treated mice.
To further identify the molecular mechanism associated with the recovery capability of the aging brain by PSPC treatment following D-gal-induced injury, we utilized the western blot analysis and biochemistry evaluation to study the relative expression of representative protein or enzyme or metabolic product-associated aging in mouse brain. Brain aging is accompanied by behavioral deficits, including cognitive and motor performances. These deficits are probably induced by oxidative damage and inflammation [32
]. The excessive ROS produced would speed up the breakdown of biomacromolecules. thus the body could be subjected to the oxidative damage. However antioxidant enzymes can greatly attenuate oxidative damage, providing support for the free radical theory of aging [40
]. D-gal could remarkably cause the decrease of antioxidant enzyme activity such as Cu/Zn-SOD and CAT () and the accumulation of ROS, whereas PSPC could restore the activity of Cu/Zn-SOD and CAT and significantly enhanced the antioxidant capacity of the body. The increased MDA content is an important indicator of oxidative damage in brain [40
]. We also found that PSPC decreased the generation of MDA induced by D-gal and reduced the production of the free radical. Therefore, PSPC retarded the aging rate induced by D-gal through the alleviation of oxidative damage.
Oxidative damage is one of the main factors of brain aging. The studies also suggest that brain aging is associated with the neuroinflammation. Evidence indicated that oxidative stress and AGEs induced by D-gal both caused neuroinflammation [41
]. Neuroinflammation had been connected to the activation of inflammatory factors iNOS and COX-2, the upregulation of GFAP expression, and the promotion of nuclear translocation of NF-κ
]. Figures –and indicated that D-gal-treatment caused inflammatory response, in which iNOS, COX-2, and GFAP levels were increased, and NF-κ
B activity was also upregulated. It was remarkable that PSPC could reverse the injurious effect of D-gal. This kind of inhibitory function from PSPC is likely to be due to the impactful effect of the anthocyanins on the inflammation. Pergola et al. found that cyanidin from the blackberry extract effectively brought down the expression of iNOS protein induced by proinflammatory stimuli bacterial lipopolysaccharide (LPS) [48
]. The other report revealed that cyanidin inhibited the expression of COX-2 and NF-κ
B induced by LPS as well [6
]. Our results indicated that PSPC not only downregulated the expression of COX-2 and iNOS protein but also inhibited nuclear translocation of NF-κ
B which was associated with the decreased AGEs levels (data not shown). So PSPC significantly attenuated the progressive inflammation induced by D-gal in mouse brain. Actually, NF-κ
B was a transcriptional regulator that could control the expression of iNOS gene and COX-2 gene in modulating inflammation and oxidative stress associated with brain aging [49
B signaling system is one of the main stress signaling pathways and/or central mediator in response to oxidant damage [47
]. The results suggested that PSPC showed anti-inflammatory effect in the neurotoxin pathway due to its strongly modulating the COX-2 and iNOS expression by reducing NF-κ
B activation. And PSPC ameliorated D-gal-induced brain impairment partly by inhibiting NF-κ
B signaling pathway.
Our group previously found that the impairment of memory and the loss of synaptic protein associated with memory formation in D-gal-treated mice may be improved by treatment with PSPC. Similarly the present study further demonstrated that PSPC could attenuate the impairment of memory and the changes of inflammation and aging factors in D-gal-treated mice. The results suggested that PSPC might improve the behavior performance of the mice not only by increasing the level of the synaptic proteins but also by enhancing the antioxidant ability which could resume activity of antioxidant enzymes, attenuate lipid peroxidation and result in inhibiting inflammatory response in mouse brain.
In conclusion, the present study demonstrated that PSPC administration attenuated D-gal-induced aging-related changes in mouse brain. PSPC increased the activity of Cu/Zn-SOD and CAT, decreased the expression of iNOS and COX-2, downregulated the expression of GFAP and MDA content, and inhibited nuclear translocation of NF-κB. PSPC consequently improved the spontaneous behavior and cognitive performance and enhanced the capacity of the brain inherent antioxidant or anti-inflammation. These findings about the pharmacological efficacy of PSPC would contribute to brain aging research or aging-related diseases research.