We found ICV Injection of Hcy is associated with high Aβ42 and γ- Secretase levels of the hippocampus. Also, aerobic training and Omega-3 intake lowered Aβ42 and γ- Secretase levels of the hippocampus in AD subjects.
It is well established that elevated plasma homocysteine and disturbed homocysteine metabolism are risk factors for AD [1]. However, the exact pathophysiological mechanisms linking high homocysteine levels with AD are not yet clear. Several potential mechanisms resulting in harmful effects of this amino acid in the brain have been proposed, including oxidative stress [41], cerebrovascular damage [42], DNA damage [43], and activation of N-methyl-D-aspartate receptors [44]. Several showed that disturbed homocysteine metabolism is related to increased CSF levels of sAPP forms and Aβ42, and may contribute to the accumulation of amyloid pathology in the brain through increasing γ-secretase pathway [43, 45]. For example, Lin et al (2009) show that Hcy increases the production of Aβ possibly by increased expression of APP as well as induction of hypomethylation of APP and PS1 gene promoters [46], whereas in rats hyperhomocysteinemia increases cerebral Aβ production by phosphorylation of amyloid precursor protein and enhancing expression of γ-secretase [47]. More recently, mice with diet-induced hyperhomocysteinemia were shown to have elevated brain Aβ levels and amyloid deposition and it was suggested that this association is mediated by the activation of the γ-secretase pathway [48]. These previous reports provide a possible explanation of the biochemical process, connecting disturbed homocysteine metabolism and increased CSF levels of sAPP forms and Aβ42.
Based on the evidence, both microglia and astrocytes secreted Aβ protein [49]. Senile plaques, mainly composed of peptides Aβ is that its main role has widely proven in the pathogenesis of Alzheimer's disease [49, 50]. In particular, increased levels of Aβ deposition in plaques outside the cell and cause synaptic dysfunction, neuronal network dysfunction, mitochondrial dysfunction, neuronal cell death, and memory loss [49, 51, 52]. Although the mechanism of neurotoxicity caused by Aβ is not yet clear, but widely proven that the accumulation of Aβ peptide in the brain causes induction of oxidative stress and neuroinflammation [49, 50]. In vitro studies have shown that injection of Aβ42 in primary hippocampal neurons leads to increased planting in the indices of oxidative stress and neurotoxicity [53, 54]. This peptide associated with oxidative stress caused by Aβ, adding vitamin E as an antioxidant, significantly dampens the effects of oxidative stress and neurotoxicity induced Aβ42 [53], which suggests that the ability of the peptide Aβ42-induced neurotoxicity toxic oxidative stress-mediated. This regard indicated that Aβ40 injection into the brain of rats is associated with the induction of free radical damage and changes in antioxidant defense such as glutathione depletion in the prefrontal cortex and hippocampus of rats [55].
It has been shown that Aβ acts as an inflammatory agent and causes inflammatory mediators such as cytokines and complement components activated brain areas to increase the risk of Alzheimer's. Besides, the presence of activated microglia and astroglia around senile plaques supports the role of Aβ in inflammation. Microglia and astrocytes may have a neuroprotective role by swallowing and clearing Aβ aggregates in the brain [56, 57]. Although it can mediate neurotoxicity effects through the release of pro-inflammatory cytokines, chemokines, ROS, and protein supplements [56]. Besides, APP expression by IL-1 (as an inflammatory cytokine) is increased and thus enhancing amyloidosis and leads to a vicious cycle [58]. Now, there is growing evidence that low to moderate-intensity training is an important factor for neural degenerative diseases [59].
In the present study Aβ42 levels in the hippocampus of AT group were significantly lower than the AC group and no difference was observed between the levels of the Aβ42 hippocampus of AT, AO, AOT, and HC groups.
The amyloid cascade hypothesis and the law of mass action, each level balance between the synthesis and degradation of proteins in the body/cleans it and Aβ levels in the brain balance between the synthesis and degradation/clean it [16].
In the present study to study changes in Aβ production in the training groups, the γ- secretase was evaluated and it was found that the level of γ- secretase of AC group was significantly higher than HC group. Thus increasing Aβ42 levels in the hippocampus of Alzheimer's rat compared to a healthy rat can be caused by increased levels of γ- secretase in hippocampal them.
The results of this study showed that the level of γ- secretase in the hippocampus of AT and AOT was significantly lower than the AC group. The results showed that there was no significant difference between Neprilysin hippocampus levels of research groups. As mentioned before, Neprilysin is considered as the main degrading enzyme Aβ and as a regulator has raised concentrations of Aβ in the brain functional surfaces [60, 61]. Failure to raise the level of Neprilysin means no change in the demolition/clean Aβ42 from the hippocampus of Alzheimer's subjects which seeks to increase the level of the index and can lead to the development of Alzheimer's disease risk. Kang and Cho (2014) examined the effect of 6 weeks of treadmill training on insulin signaling and brain Aβ levels in the Alzheimer induce rat by streptozotocin. Their results showed a significant decrease in Aβ42 levels and increase insulin signaling in the brains of rat of Alzheimer training compared to Alzheimer control groups. They suggested that reducing insulin signaling is associated with elevated levels of γ- secretase, which leads to an increase in Aβ and improves insulin signaling caused by six weeks of training on a treadmill, might be a moderation of γ- secretase to reduce Aβ [62]. Also, Liu et al (2013) showed that the number and size of Aβ plaques in the hippocampus of a rat with Alzheimer's disease five months after the treadmill training was significantly reduced. The levels of Aβ42, tau protein, and PS1 expression decreased significantly as a result of training on the treadmill. Besides, a reduction in the levels of CTFs and sAβPPβ in training transgenic rats was observed. The researchers concluded that perhaps treadmill workouts prevent the amyloidogenic pathway and the breakdown of APP to be adjusted to non-amyloidogenic[63]. Also, Kang et al (2013) stated that 12 weeks of treadmill training to prevent the disorder gene mutation PS2 and reduced the accumulation of Aβ by inhibiting the activity of β- secretase and its products [64]. Besides, Um et al (2008) also showed that 16 weeks of treadmill training causes a significant decrease in Aβ42 in the brain of rats with Alzheimer's protein levels [65]. The results also showed that training with two different intensities by reducing the γ- secretase that is amyloidogenic pathway causes a decrease in Aβ that is consistent with the results obtained from the research.
There are several theories about changes in Aβ levels and metabolism (synthesis and degradation) as a result of training. According to the physical activity mediates many gene products both at the mRNA and protein level, Inducer of anatomical changes, chemical and electrophysiological nerve, is enhancer the plasticity of neurons [66], multiple paths are likely enabled to adjust the level of Aβ directly or indirectly. Adlard et al (2005) stated that exercise training can probably mediate the metabolism of APP and Aβ cascade in the brain to reduce the production of Aβ (decreasing amyloidogenic activity) which is independent of the neprilysin and IDE [67]. The second possibility is that exercise directly modulates the APP metabolism by increasing the activity of neurons. For example, processing of APP can be completed by mitogen-activated protein kinase (MAPK) and phospholipase C and has proved that these pathways are activated through exercise [68]. On the other hand, cholinergic activity increases with exercise and cholinergic systems by physical activity involved in neuronal plasticity induced by exercise [66]. Mentioned is probably exercise can improve behavioral disorders by reducing the number of peptide Aβ42 by increasing the production of neurotrophic factors (NGF, BDNF, and IGF-1) which are important for neuronal survival, a proliferation of neuronal and synaptic plasticity [69]. Also, physical activity reduces Aβ plaque and improves spatial learning (three-dimensional), memory, synaptic plasticity, and nerve tissue of Alzheimer's disease rat [70]. Several studies have reported that the cytoplasmic surface of apoptotic markers such as Cytochrome C, caspases-9, caspases-3, and Bax protein was significantly decreased in the brains of active Alzheimer's rat to the inactive Alzheimer's rat. In Alzheimer's rats, decreased pro-apoptotic proteins, including cytochrome C and boxes with physical activity, possibly by preventing apoptotic pathways related to caspases, are systematically associated with lower levels of protein Aβ [65, 71]. However, Park et al (2010) has provided a cyclic process stimulate Aβ in inflammation and suggested that signaling TNF-α, ultimately leads to the production of Aβ peptides causing produce new pathogenic Aβ peptides and increase its production and thus leads to Alzheimer's disease that can create a stronger cycle [72]. In this regard, Nichol et al (2008) demonstrated that inflammatory markers (IL-1β and TNF-α) in the hippocampus of Alzheimer's transgenic rat was higher than the healthy rat and the level of anti-inflammatory agents (IFN-γ and MIP-1α) in Alzheimer's transgenic rat was lower than the healthy rat. After 3 weeks of training, the levels of IL-1β and TNF-α decreased and was close to the normal group that this reduction was associated with an increase in IFN-γ and MIP-1α. Also a significant decrease in the levels of Aβ40 solution and fibrillar Aβ solution in the cortex of Alzheimer's transgenic rat by 3 weeks optional practice was observed [73]. Also, Um et al (2008) showed that the SOD-1 protein and catalase in the brains of active Alzheimer's rat has a significant increase compared to the inactive. Exercise causes increased levels of these defense indexes that these changes are associated with reduced apoptotic protein (Cytochrome C, caspases-9, caspases-3, and bax) and an increase in Hsp70 and BDNF which is induced by regular physical activity in the brain and then was mediated by peptides Aβ42 clinical reduced in rat with Alzheimer's disease [65]. It has been shown that 16 weeks of treadmill exercise combined with α-lipoic acid, decreased levels of brain Aβ42 transgenic Alzheimer's rat. The researchers reported that increased oxidative stress, is one of the main factors involved in Alzheimer's which leads to increased production of ROS and causes the destruction of cellular structures, and ultimately apoptosis increased the production of Aβ. Exercise alone and in combination with α-lipoic acid supplementation resulted in increased levels of oxidative stress and antioxidants as immunosuppressive agents and finally resulting in reduced apoptotic index and Aβ [71]. Possible mechanisms in this regard include the reduction of oxidative stress and increase antioxidant defense enzyme activity act as an agent enhancing the α-secretase and inhibitor β- and γ- secretase and so the processing of APP to conduct the Non-amyloidogenic pathway [74]. The improvement in Aβ and γ- secretase levels in the present study may also be due to the reduction of oxidative stress and the improvement of antioxidant defense through omega-3 supplementation and aerobic exercise.