As a synthetic GC, DEX impairs the structure and function of the hippocampus, ultimately contributing to AD progression (31). In this study, we expanded upon our previous research by investigating the effects of DEX on memory impairment and neuropathology in APP23/MAPT mice. Our findings revealed that DEX exacerbated behavioral deficits by reducing SPs burden and tau phosphorylation. In addition, DEX reduced Aβ degradation and clearance in APP23/MAPT mice. Moreover, we found that NLRP1 inflammasome played key roles in AD regulation by DEX. Interestingly, LEV suppressed neuroinflammation by deactivating inflammasomes and protected neurons from dystrophy and loss by suppressing DEX-induced apoptosis.
DEX was suspected to impair learning and memory abilities. As anticipated, DEX increased the time taken and distance traveled by mice to find the platform and decreased crossing times (Fig. 1). Similar to our findings, DEX reportedly exacerbated Aβ-induced learning and memory impairment in rats in a previous study (32). In another study, DEX was confirmed to induce memory and learning impairment in senescent mice (33). In addition, melatonin could reportedly attenuate DEX-induced spatial memory impairment and DEX-induced reduction of synaptic protein levels in the mouse brain (34). In line with these observations, our results demonstrated that LEV could ameliorate AD progression by inhibiting Aβ production and deposition and tau phosphorylation in the brains of DEX-treated APP23/MAPT mice (Figs. 1–3). Although no related associated study exists, LEV reportedly normalizes hippocampal CA3/DG activity and improves memory performance in patients with amnestic mild cognitive impairment based on the results of high-resolution functional magnetic resonance imaging techniques (35). In addition, hippocampal spatial memory decline in aged rats was rescued by chronic infusion or a single injection of LEV and sodium valproate before training, as reflected in the MWM test performance (36). In a previous study including APP/PS1 mice, LEV effectively alleviated behavioral deficits in AD (37). Consistent with these findings, LEV administered intraperitoneally improves neuronal functions in APP/PS1 Tg mice, suggesting its ability to penetrate the blood-brain barrier (37). Likewise, LEV reversed the abnormal expression of neuronal activity-related proteins that reflect hippocampal remodeling and cognitive deficits in hAPPJ20 mice (38). Therefore, these results suggest the roles of DEX in exacerbating the pathological features of AD, which were ameliorated by LEV in the brains of APP23/MAPT mice.
Considering that Aβ production and deposition are widely accepted as key pathogenic factors of AD (39), we initially investigated the effect of DEX on Aβ production and deposition. The results indicated that DEX induced Aβ production and deposition in the brains of APP23/MAPT mice (Fig. 2). In agreement with our finding, chronic DEX treatment accelerates Aβ production in the neurons of APP/PS1 mice (24). DEX further induces Aβ production with cholinergic dysfunction similar to AD (40). Aβ results from β-site APP-cleaving enzyme 1 (BACE1) and γ-secretase-mediated cleavage of APP. Consistently, we found that DEX elevated BACE1 and PS1 levels responsible for Aβ production in the brains of APP/MAPT mice (Fig. 2). Similarly, GC exposure elevated Aβ production by increasing BACE1 and APP gene expression in primary astrocytes (41). In addition to inducing Aβ production and deposition, DEX induces tau hyperphosphorylation (40), which is consistent with our results (Fig. 3). GC exposure induces tau hyperphosphorylation and neurostructural deficits in hippocampal neurons in vitro and in vivo (42).
To counteract the side effects of DEX, LEV was used to treat APP23/MAPT mice. The results demonstrated that LEV suppressed Aβ production and deposition and tau phosphorylation (Figs. 2 and 3). Although no relevant research exists, LEV and TPM reportedly reduce Aβ production by inhibiting the activity of γ-secretase (37). In addition, the underlying mechanisms may involve Aβ uptake and degradation (43). In line with our findings, the lack of Aβ uptake by glial cells is a major cause of sporadic AD (44), which might be caused by impaired Aβ clearance in AD (45). Regarding the underlying mechanism, LEV increases Aβ uptake (Fig. 7a). Aβ degradation is regarded as a new therapeutic target for AD treatment (46, 47). Our study contributed to previous studies showing that LEV enhanced Aβ degradation (Fig. 7b). Moreover, tau phosphorylation can be induced by Aβ production and deposition (48). We found that LEV decreased tau phosphorylation via CDK5- and GSK3α/β-dependent mechanisms (Fig. 3). Similarly, tau phosphorylation is reportedly mediated by CDK5 and GSK3α/β (49, 50). Based on these findings, we can infer that LEV counteracts the effects of DEX on Aβ production and deposition and tau hyperphosphorylation.
Excessive Aβ loading and tau hyperphosphorylation result in neuroinflammation and neuronal loss in AD (51). Inflammasomes contribute to various disorders in the CNS by causing neuroinflammation. Extracellular accumulation of Aβ in SPs in brains with AD is a principal event in AD (52). Deposition of Aβ peptide initiates inflammasome activity in the microglia (53). Moreover, inflammasome activation causes AD impairment through Aβ deposition and loss of spatial memory via harmful chronic inflammatory response. It is important to note that NLRP1 activation in the brain is restricted to plaque-associated microglia, suggesting that microglial activation of the NLRP1 inflammasome is a pivotal event in AD pathogenesis (54). Furthermore, the correlation between AD and local neuroinflammation has been established. As an important inflammasome component, IL-1β can induce tau phosphorylation (55), leading to compromised learning and memory in animals with AD (56). By blocking IL-1β, AD was ameliorated in animals with AD (57). Consistent with other studies, our study further shows that LEV deactivates DEX-induced inflammasomes, thus suppressing glial cell activity (Fig. 6a). The glial cells are responsible for Aβ clearance, whereas the microglial cells are responsible for degrading Aβ degradation via autophagy (58).
Accumulating evidence has indicated that elevated levels of GC cause chronic environmental stress, which is a risk factor for AD (59). Evidence also suggests that high but not low levels of DEX exposure impair hippocampal neurons in rats (60). Using SH-SY5Y cells, high levels of DEX induce neuronal loss and neurotoxicity by impairing the mitochondria (61). Consistent with previous studies, we further found that DEX inhibited the expression of both NeuN and SYP, leading to the loss of neurons in the synapse (Fig. 4c). Moreover, the apoptotic mechanisms underlying the above process were identified (Fig. 5). LEV attenuated DEX-induced apoptosis, ameliorating neuronal and synaptic loss (Figs. 4c and 5). Our previous study had shown the protective effects of LEV on neurons via apoptosis inhibition in kainic acid (KA)-activated APP23/MAPT mice (25). In support of our findings, LEV administration after hypoxia reduces neuronal apoptosis in a neonatal rat model of hypoxic-ischemic brain injury (62). Additionally, a study showed that LEV conferred neuroprotective effects against focal cerebral ischemia-reperfusion injury in mice (63).
In conclusion, this study builds on previous studies by showing that LEV ameliorated amyloidosis and tauopathy by inhibiting the activity of NLRP1 inflammasome in mice with DEX-induced AD.