Metal ions are required for maintaining the functions of many proteins and proper metal ion balance in the brain is significant for normal cognitive function . Thus, metal ions have received exponentially increasing interest. Growing evidence has been collected on the relationship between metal ions and the development of neurological disorders, such as metal-protein association inducing protein aggregation and metal-catalyzed protein oxidation inducing protein damage and/or generation of reactive oxygen species (ROS) [23, 24]. Metals such as Al, Fe, Cu, and Zn were dysregulated in AD brain tissue to create a pro-oxidative environment [25–29]. In the frontal cortex of young and aged rats fed with AlCl3, the Al, Fe and Zn contents significantly increased and Al may be linked with alteration in neurobehavioral activity . The multifunctional metal-ion chelators as a potential treatment for metal-promoted neurodegenerative diseases (MpND) has attracted much attention and showed promise of disease-modifying [31–34].
Al as an important neurotoxin has been investigated extensively both in vitro and in vivo, and is associated with cognitive dysfunction and various mental diseases. Recent neuropathological, biochemical, and epidemiological studies suggest that Al contributes to the progression of several NDDs, including AD, and PD, but the precise mechanism has not been clarified yet [30, 35–37]. Intracerebroventricular (icv) injection of trace AlCl3 into mice will result in neurodegeneration and learning/memory disorders . However, oral ingestion is the main form of Al exposure in clinic. Because the icv animal models do not much resemble that from oral ingestion of Al, several scientists hold that the icv AlCl3 injection model does not strictly speak a neurodegeneration model. In the present study, we established neurodegenerative models by intragastric administration of aluminum gluconate (200 mg Al3+ · Kg−1, once a day, 5 d a week, for 20 weeks) [18, 19]. The results showed that the SLM function was significantly impaired and significant karyopycnosis of hippocampal neurons was observed in the model group compared with the control group.
Al neurotoxicity may be related to the integrity and permeability of BBB . Al can induce apoptosis in rat hippocampal cells through the down-regulation of bcl-2 mRNA expression and the up-regulation of bax mRNA expression . Al may also be involved in the aggregation of Aβ peptides, inducing Aβ peptides into the β-sheet structure and facilitating iron-mediated oxidative reactions . Neurodegeneration caused by aluminum overload was associated with an imbalance in metal ion levels in the brain. Metal dyshomeostasis is linked in protein misfolding and may contribute to oxidative stress and neuronal damage. The presence of Al might change the contents of endogenous trace metals .
Iron as an important trace element is essential for neuron development since it is required for various physiological events, including mitochondrial respiration, oxygen transport and DNA synthesis . However, iron contributes to oxidative stress through Fenton reaction, leading to damages in DNA, proteins and membrane [44, 45]. Iron imbalance is a precursor to the neurodegenerative processes leading to AD , and quantification of brain iron content can be an effective marker for early diagnosis of AD . Iron accumulation may contribute to protein aggregation and neuronal death in PD patients . Excessive iron would induce cell injury by reacting with H2O2 to produce hydroxyl radical (OH−), superoxide anions (O2−), and ROS . Another hypothesis states that iron-mediated free radical production contributes to BBB opening to cause neuronal damage . In our study, the iron content in the model group was significantly higher compared with the control group, and iron content was the highest among the tested metals, implying that iron overload in hippocampus may play an important role in the occurrence of neuron damage.
Other transitional metals such as Mn, Cu, and Zn are essential enzyme cofactors required for numerous cellular processes, but their abnormal accumulation in the brain will lead to neurotoxicity . Mn has long been known to cause neurological disorders similar to PD. Mn might result in movement abnormalities in PD patients . The present study revealed that Mn content in hippocampus of the model group was 8.8 times (the highest ratio) higher compared with the control group. The mechanism of Mn-induced neurotoxicity has not been fully elucidated, but an established mechanism is correlated with attenuated uptake of glutamate (GSH) . Mn can reduce brain glutathione level, likely reflecting oxidative stress , and might lead to mitochondrial dysfunction and trigger apoptotic-like neuronal death . These studies indicate that the obvious increase of Mn content in hippocampus may play a key role in the mechanism of chronic Al-induced brain damage and neural degeneration.
Cu which is released at the synaptic cleft is an important structural cofactor in a series of biochemical processes with a narrow-range of optimal content . The knowledge of Cu homeostasis has become increasingly important in clinical medicine, as it can be involved in the pathogenesis of NDDs such as AD [56–59]. The mechanism may be that Cu affects the degradation and aggregation of Aβ in AD [60, 61]. We found that Cu content significantly increased after 20-week administration of aluminum gluconate, and this may be a reason for the SLM function impairment and neuron death.
Zn, essential for human health in trace amounts, is co-released with GSH and the significance of Zn signaling is gradually recognized . Hippocampal pyramidal neurons are vulnerable to brain injury, while Zn entry may enhance this vulnerability . Zn has been implicated in AD and PD. Excessive Zn translocation might be a molecular trigger of the cellular apoptosis [64, 65]. In our experiments, the hippocampus of model rats showed Zn accumulation, and we thought that Zn is also involved in the occurrence of brain injury.
Neurons in brain are highly sensitive to oxidative stress. Metal toxicity is a problem leading to oxidative stress. Superoxide radicals can also create further oxidative stress by metal-catalyzed reactions . SOD converts superoxide to H2O2 and oxygen. SODs are the most important antioxidant enzymes in the antioxidant defense system . MDA is an end-product of lipid peroxidation and an excellent marker for degeneration of neurons . Besides, metal ion contents in hippocampus of the model group significantly increased compared with the control group. The hippocampal SOD activity was weakened and MDA content increased both significantly in the model group. The results might further confirm the hypothesis that imbalance of cerebral metal ion is involved in occurrence of oxidative stress.
Moreover, meloxicam could significantly suppress metal ion elevation and prevent hippocampal neuron injury in aluminum overload rats. Reportedly, COX-2-induced synthesis of prostaglandins (PGs) was associated with chronic inflammation [68, 69], causing oxidative stress. Our previous study showed that chronic aluminum overload significantly elevated COX2 mRNA and protein expressions . These results suggest that as a selective COX2 inhibitor, meloxicam might alleviate oxidative stress damage to the brain by inhibiting COX2 activity, relieving inflammation and reducing metal ion imbalance. It may be involved in the neuroprotective mechanism of meloxicam against rat hippocampal neuronal injury following chronic aluminum overload.
In conclusion, we provide evidence that metal ion imbalance may contribute significantly to hippocampal injury caused by exposure to aluminum. Meloxicam was neuroprotective by decreasing COX2 expression and was associated with inhibition of oxidative stress. Clearly, further studies are necessary to clarify the neuroprotective mechanisms of meloxicam after exposure to aluminum.