Date Published: March 04, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Chaoxiu Ren, Xiangang Hu, Qixing Zhou.
Both oxidative stress and neurotoxicity are huge challenges to human health, and effective methods and agents for resisting these adverse effects are limited, especially in vivo. It is shown here that, compared to large graphene oxide (GO) nanosheets, GO quantum dots (GOQDs), as nanozymes, efficiently reduce reactive oxygen species (ROS) and H2O2 in 1‐methyl‐4‐phenyl‐pyridinium ion (MPP+)‐induced PC12 cells. In addition, GOQDs exert neuroprotective effects in a neuronal cell model by decreasing apoptosis and α‐synuclein. GOQDs also efficiently diminish ROS, apoptosis, and mitochondrial damage in zebrafish treated with MPP+. Furthermore, GOQDs‐pretreated zebrafish shows increased locomotive activity and Nissl bodies in the brain, confirming that GOQDs ameliorate MPP+‐induced neurotoxicity, in contrast to GO nanosheets. GOQDs contribute to neurotoxic amelioration by increasing amino acid metabolism, decreasing tricarboxylic acid cycle activity, and reducing steroid biosynthesis, fatty acid biosynthesis, and galactose metabolic pathway activity, which are related to antioxidation and neurotransmission. Meanwhile, H2O2 decomposition and Fenton reactions suggest the catalase‐like activity of GOQDs. GOQDs can translocate into zebrafish brains and exert catalase‐mimicking activity to resist oxidation in the intracellular environment. Unlike general nanomaterials, biocompatible GOQDs demonstrate their high potential for human health by reducing oxidative stress and inhibiting neurotoxicity.
Neurotoxicity has severe influences on human health and has pathological features, such as memory disorders, learning ability decline, behavioral dysfunction, and cognitive dysfunction.1 Although several therapeutic agents, such as epigallocatechin‐3‐gallate, anthocyanins, sialic acid, and thioflavin, have been proposed to mitigate neurotoxicity, poor chemical stability, loss of activation in harsh environments, and substandard bioavailability limit the use of these agents.2 Nanomaterials have several unique physicochemical characteristics for overcoming the above drawbacks: a small size for crossing the blood–brain barrier (BBB); an ultrahigh surface‐area‐to‐volume ratio and specific surface chemistry for enhancing biocompatibility and binding affinity; an increased resistance to biodegradation; a decreased susceptibility to denaturation; and an ability to mimic the cellular matrix in natural tissues or body fluid, in a manner that is similar to enzymes and proteins, for therapeutic applications.3
ROS such as H2O2 and •OH are highly reactive and can modify intracellular molecules, disrupt the redox balance, and subject the cells to oxidative stress,22 which induces various biological responses, including neurotoxicity.23 Our recent study showed that GO nanosheets translocated into the brains of zebrafish and induced oxidative stress and neurotoxicity.[[qv: 5b]] Overproduction of ROS induced by MPP+ was mitigated by pretreatment with GOQDs in PC12 cells and the brains of larval zebrafish. The apoptosis, mitochondrial damage, and senescence induced by MPP+ were also mitigated by pretreatment with GOQDs. In addition, previous reports have shown that senescence is accompanied by a decline in behavioral functions,[[qv: 3c,23d]] which is a defining characteristic of neurotoxicity.[[qv: 5b,24]] Behavioral impairments in larval zebrafish were mitigated by pretreatment with GOQDs, suggesting that GOQDs provided neuroprotection. Experimental data have also shown aggregation of α‐synuclein in MPP+‐induced cells, which is a major component of Lewy bodies, the pathological hallmark of neurodegeneration.25 Pretreatment with GOQDs reduced the expression of α‐synuclein, further confirming that the GOQDs mitigated neurotoxicity. Meanwhile, the major changes in the Nissl bodies, including the dissolution and disappearance of the Nissl bodies, were associated with neuronal injury.20 Nissl body expression increased in the GOQDs‐pretreated group, resulting in an increase in the healthy neurons observed in the zebrafish brains. GOQDs reduced the H2O2 and ROS overproduction induced by MPP+ in PC12 cells and larval zebrafish by translocating into the cells and brains,26 which suggested that the GOQDs retained their catalase‐like activity in the intracellular environment. The catalase‐like activity probably contributed to the protective roles of the GOQDs. Recently, the intrinsic catalase mimetic activity of iron oxide nanoparticles, vanadium oxides nanoflakes, and gold nanoclusters has also been identified.[[qv: 3c,27]]
Our recent study showed that GO nanosheets translocated into the brains of zebrafish and induced oxidative stress and neurotoxicity.[[qv: 5b]] Compared with GO, GOQDs protected PC12 cells and larval zebrafish from neurotoxicity; the underlying mechanisms are shown in Figure S10 (Supporting Information). In vitro, the GOQDs efficiently reduced MPP+‐induced ROS, H2O2, cell toxicity, apoptosis, and SA‐β‐Gal expression. The GOQDs protected against neurotoxicity by inhibiting the expression of α‐synuclein. In vivo, the GOQDs efficiently diminished MPP+‐induced ROS, mortality, the malformation rate, apoptosis, mitochondrial damage, and SA‐β‐Gal expression. Increased locomotive activity and Nissl bodies were observed in the brains of GOQDs‐pretreated larvae and further confirmed that the GOQDs mitigated neurotoxicity. The enhancement of amino acid metabolism and inhibition of the TCA cycle, steroid biosynthesis, fatty acid biosynthesis, and galactose metabolism by GOQDs, compared with MPP+, may participate in fundamental metabolic pathways related to antioxidation and neurotransmission and contribute to neurotoxic amelioration. Meanwhile, the decomposition of H2O2 and Fenton reactions implied the catalase‐like activity of the GOQDs. The present study indicates that GOQDs can mitigate neurotoxicity in vitro and in vivo via antioxidative activities and metabolic regulation.
Characterization of GOQDs is described in the Supporting Information.
The authors declare no conflict of interest.