In the present paper, we found SV2A to be colocalized with APP and downregulated in hippocampus of AD patients (Fig. 1), Moreover, we showed that abrogation of SV2A promotes Aβ production (Fig. 2), and upregulation of SV2A downregulates the AD risk factors BACE1 and APOE (Fig. 3). Further, SV2A deficiency promotes Tau hyperphosphorylation (Fig. 4). Finally, SV2A regulation of the pathogenesis and development of AD appears to be mediated by the PI3K signaling pathway (Figs. 5). This study provides guidelines and information regarding the mechanism of SV2A influence in the regulation of AD and possible future research of neurological diseases.
Indeed, synaptic loss is a prominent AD pathology and a major structurally related factor for AD cognitive impairment[16, 40].SV2A is a synaptic vesicle protein with 12 transmembrane domains ubiquitously expressed in the central nervous system and has brain specificity. It has been shown that people with AD had significantly less SV2A binding in the hippocampus compared to cognitively normal participants according to SV2A PET using 11C-UCB-J and 18F-UCB-H [16, 27]. The hippocampus is a critical part of the brain for memory formation, and an important site of early damage associated with Alzheimer's disease . Here, we found a significant decrease of SV2A expression at mRNA level in the hippocampus of AD patients comparing with non-AD subjects as per the Allen Brain database analysis (Fig. 1), indicating that changes in SV2A expression influence synaptic function in AD.
It has been shown that the expression of SV2A was down-regulated in the hippocampus of AD patients, suggesting that SV2A may serve as a key regulator to AD related proteins. such as Aβ and APP, two hallmarks of AD . Our study detected an increase in the expression levels of APP in APPswe293T cells infected by SV2A silencing virus (Fig, 2A, B). The results of immunofluorescence analysis of APP and SV2A (Fig. 2C), ELISA statistics analysis of the Aβ (Fig. 2D), and the result of brain observation by PET in SV2A KO mice (Fig. 2E) also validated the increase of APP in SV2A deficiency situation. It has been well documented that APP is stabilized by the GABAB receptor, at the cell surface, which leads to a reduction in APP proteolysis to Aβ. Meanwhile, it has also been demonstrated that SV2A deficiency impairs its interaction with synaptotagmin 1 causing a specific disruption of synaptic GABA release, which in turns downregulates GABAB receptor expression . Therefore, we speculated that down-regulation of SV2A may impair the inhibition of APP proteolysis to Aβ via reducing the expression of the GABAB receptor, thus leading to up-regulation of APP and Aβ. Yet there is also evidence showing that in AD, the SV2A loss takes precedence in glutamatergic rather than GABAergic nerve terminals, and the specific regulatory mechanism of SV2A on APP remains to be elucidated.
A previous study had inferred that APP and Aβ may be regulated by SV2A. In the present study, we found that SV2A overexpression downregulates BACE1 and APOE at both the mRNA and protein expression levels (Fig. 3). The depletion of BACE1, which initiates Aβ production by cleaving the extracellular domain of APP , has been reported to restore part of the synaptic function, indicating that BACE1 may be necessary for optimal synaptic activity and cognition. APOE is a major risk factor for Alzheimer's disease (AD). Studies have shown that the level of BACE1 in the brain may be affected by APOE before the onset of AD[50, 51]. APOE allele 4 has been associated with an increased risk of AD. In addition, APOE regulates multiple brain pathways to varying degrees including lipid transport, synaptic integrity and plasticity, and cerebrovascular function . It has been reported that SV2A knockout appears to negatively regulate dendritic spine density and synaptic plasticity . SV2A regulates the release of action potential dependent neurotransmitters and SV2A dysfunction impairs the release of synaptic GABA and glutamate  . Inhibition of BACE1 can promote the activities of various cell receptor proteins in presynaptic or postsynaptic glutamatergic and GABAergic synaptic membranes. Therefore, we can speculate that SV2A can further influence AD by mediating BACE1 and APOE to regulate synaptic receptor. However, some studies have shown that SV2A knockout prevents ApoE4 from promoting BACE1 processing of APP which plays role in WT cells by promoting the co-localization of BACE1 and APP in vivo 
It has also been reported that SV2A density was inversely correlated with Tau phosphorylation. Therefore, we investigated the relationship between SV2A and Tau, and found that SV2A overexpression in APPswe293T cells resulted in significant decreases in Tau mRNA and protein levels (Fig. 4A). Although p-Tau-S356 and p-Tau-Y322 did not change significantly with SV2A overexpression, a slight upward trend was found in p-Tau-S356 of shSVA2 (Fig. 4B, C). Meanwhile, SV2A knockout led to significantly increased levels of Tau protein phosphorylation (Fig. 4D). It is known that Tau cross-links synaptic vesicles, thereby slowing their mobilization and ultimately reducing synaptic transmission during intense stimulation . Tau-induced deficits in synaptic vesicle mobility prevent their recruitment into release and thus lead lower neurotransmission . Further, overexpression of Tau is found to potently inhibit axonal transport . Since SV2A is involved in the regulation of synaptic vesicle transport, exocytosis, and neurotransmitter release , overexpression of SV2A may alleviate AD-related symptoms by reducing the content of tau and thereby inhibiting the impairment of synaptic vesicle information transmission.
SV2A is a synaptic vesicle membrane protein involved in the regulation of neurotransmitter release, and thus is essential for the activation of crucial signaling pathways . In this study, we found that the involvement of SV2A in the pathogenesis and development of AD appears to be mediated by the PI3K signaling pathway, as upregulation of SV2A downregulated the expression of PI3K (Fig. 5A), and treatment with the PI3K inhibitor LY294002 blocked this effect (Fig. 5D). Decreased levels of PI3K subunits, as well as blunted AKT kinase phosphorylation, have been observed in the brain of AD patients . Insulin-like growth factor-2 (IGF-2) was reported to attenuates memory decline and amyloid plaques plaque formation in AD mouse model by activating the PI3K/AKT/CREB signaling pathway ., and insulin has been observed to promote neuron growth and synapse formation through the PI3K signal pathway [64, 66]. Therefore, we speculate that SV2A may play a role in promoting insulin growth factor secretion. However, further studies are needed to clarify the link between SV2A, the PI3K signal pathway, and AD pathogenesis and progression.