In this study, we demonstrated that ALA alleviated cognitive impairment and AD-related pathologies after cerebral ischemia-reperfusion injury in a rat model of tMCAO through down-regulation of GSK-3β. After transient cerebral ischemia for 2 h, cerebral infarction and brain edema were found, and consequent neurologic deficits and cognitive impairment were observed in tMCAO rats. Meanwhile, GSK-3β and microglia were activated in the hippocampus. Furthermore, increased expression of AD-related proteins (APP, BACE1, Aβ, and tau) and reduced expression of synaptophysin were found in the hippocampus 7 days after cerebral I/R injury. It was found that administration of ALA after cerebral I/R injury reduced cerebral infarction, brain edema, and improved neurologic deficits and cognitive impairment. In parallel with these findings, ALA reduced the expression of APP, BACE1, Aβ, and phosphorylated tau in the hippocampus, down-regulated the activation of GSK-3β and microglia, and improved the integrity of neuronal synapses.
To the best of our knowledge, this is the first report indicating that ALA can down-regulate the activation of GSK-3β after cerebral I/R injury and reduce AD-related pathologies, as we demonstrated in this study. It has been shown that cerebral hypoxia-ischemia can increase the production of APP and activate BACE1, and further increase the production of Aβ [5] and the levels of Tau proteins [5, 20], suggesting that cerebral ischemia can produce AD-related pathologies, including activation of BACE1 and increased production of APP, Aβ, and Tau. Indeed, our findings support this notion (Fig. 3).
ALA is an organosulfur compound displaying mainly anti-oxidant and anti-inflammatory activity [21, 22]. The application of ALA in diverse diseases, including CNS diseases, has been receiving growing attention, [23–25]. Among them, ALA has long been recognized to be protective against cerebral I/R injury [25]. The mechanisms through which ALA protects against cerebral I/R injury involve mainly its anti-oxidant and anti-inflammatory activities. In the current study, we demonstrated the anti-inflammatory protective effects of ALA against the activation of GSK-3β and microglia that may lead to cognitive impairment and AD-related pathologies after cerebral I/R injury. In fact, activation of GSK-3β and microglia play an important role in the pathological consequences of cerebral I/R injury.
GSK is a constitutive protein kinase, with multiple function involving diverse pathophysiological pathways. Physiologically, it has been found to be involved in the regulation of cell cycle, gene expression, development, and metabolism [26]. Pathologically, it is involved in psychiatric diseases, neurological diseases, cardiovascular diseases, diabetes mellitus, inflammatory diseases, bone diseases, and cancer [6]. GSK-3β activation has been found in cerebral ischemic injury [7–9]. Application of GSK-3β inhibitors protects against ischemic neuronal injury. GSK-3β activation has been noted in the pathophysiological processes leading to AD [10, 27]. Moreover, GSK-3β activation seems to be involved in the production of AD-related pathologies after cerebral ischemic injury. GSK-3β activation has been correlated with increased production of APP [28, 29] and hyperphosphorylation of Tau [28] in the neonatal rat brain after hypoxia-ischemia. Furthermore, inhibition of GSK-3β decreased the production of APP [28] and hyperphosphorylation of Tau [28, 30], and exerted its neuroprotective activity against cerebral I/R injury. In the current study, we clearly demonstrated that reduction in the activation of GSK-3β by ALA reduced AD-related pathologies, including activation of BACE1 and production of APP, Aβ, and Tau in cerebral I/R injury (Fig. 3).
Although not directly connected to cerebral ischemia, GSK-3β activation has been shown to lead to microglial activation in another inflammatory condition. Green and Nolan [31] have shown that lipopolysaccharide (LPS) can induce an increase in GSK-3β expression and activity in microglia in vitro. The current study showed that GSK-3β activation correlated well with microglia activation, since ALA down-regulated GSK-3β activation in parallel with microglia activation in cerebral I/R injury.
Microglial activation has been noted for a long time in cerebral ischemic injury [32]. The early activation of microglia in cerebral ischemic injury is detrimental to the brain. In our previous study, it was found that early administration of ALA can alleviate cerebral I/R injury in rats by down-regulating microglia activation [11]. In the current study, early administration of ALA not only alleviated cerebral I/R injury but also improved cognitive impairment and reduced AD-related pathologies through down-regulation of microglia activation.
Nevertheless, this is the first study indicating that ALA can down-regulate the expression of GSK-3β after cerebral I/R injury. Furthermore, this is also the first study indicating that ALA can reduce AD-related pathologies after cerebral I/R injury. In the current study, ALA was shown to down-regulate the activation of GSK-3β and microglia, and reduce AD-related pathologies, including the expression of APP, BACE1, Aβ, and tau after cerebral I/R injury.
Limitations
Our study is a descriptive study. It demonstrated the cascade of Alzheimer’s disease-related neuropathological changes after cerebral ischemic injury in the rat model of transient middle cerebral artery occlusion. Future work is needed in mechanistic exploration regarding how ALA regulates GSK-3β after cerebral ischemic injury. Besides, this is a in vivo study and it warrants further efforts in future application of ALA in clinical treatment. In fact, ALA has been used as either the nutritional supplement or the drug in the treatment of diabetic polyneuropathy in many countries. Hence, the translational use of ALA in the treatment of VCID can be expected at least in terms of safety issues.