We recently reported that Herpes Simplex Virus type 1 (HSV-1) induces Ca2+-mediated phosphorylation of amyloid precursor protein (APP) at Thr668 and accumulation of amyloid-β protein (Aβ) in rat cortical neurons (Piacentini et al., 2011). To assess the potential impact of HSV-1 infection on the synaptic function we investigated: i) the role of glycogen synthase kinase (GSK)-3β activation in APP phosphorylation and Aβ accumulation caused by HSV-1; ii) the effects of these intracellular pathways on expression of the presynaptic proteins, synapsin and synaptophysin.
We found that phosphorylation of GSK-3β at Tyr216 was significantly increased (+82.6%; P<0.001) in HSV-1-infected neurons. This effect was critical to APP phosphorylation at Thr668 (pAPP). Indeed, in the presence of the specific GSK-3β inhibitor, SB216763 (10 µM), pAPP was significantly reduced (-43%; P<0.001) in neurons challenged with HSV-1 for 10-60 min. Similar inhibition (-30%; P<0.001) was observed at 18-24 hours post infection (hpi). These data suggest that both virus binding to plasma membrane (occurring during 10-60 min exposures of neurons to HSV-1) and intracellular viral replication (i.e., 18-24 hpi) cause GSK-3β activation triggering APP phosphorylation. The latter is believed to be a key event in APP processing and Aβ production. Indeed, we found that the GSK-3β inhibitor, SB216763, also significantly reduced (-22%; P<0.001) the accumulation of Aβ in HSV-1-infected neurons at 24 hpi. Activation of GSK-3β may also inhibit the transcriptional program mediated by the cAMP response element-binding protein (CREB). We then checked whether CREB activity was altered in HSV-1-infected neurons. We found that HSV-1 inhibited CREB by reducing its phosphorylation at Ser133 and increasing that at Ser129. These effects were reversed by SB216763 thus showing their dependence on GSK-3β activation. Given that both CREB inhibition and Aβ may affect the synaptic transmission machinery, we investigated the effects of HSV-1 on the expression of the presynaptic proteins synapsin and synaptophysin. The expression of both proteins was significantly reduced in infected neurons (-58%; P<0.01). This reduction depended on both GSK-3β activation and Aβ buildup in neurons given that it was counteracted by cell treatments with either SB216763 or an antibody raised against Aβ (4G8).
Taken together, these findings suggest that HSV-1 alters the expression of synaptic proteins via activation of GSK-3β and intraneuronal accumulation of Aβ. These data further support the view that recurrent HSV-1 infections spreading to the CNS may contribute to the pathophysiology of Alzheimer’s disease.
We found that phosphorylation of GSK-3β at Tyr216 was significantly increased (+82.6%; P<0.001) in HSV-1-infected neurons. This effect was critical to APP phosphorylation at Thr668 (pAPP). Indeed, in the presence of the specific GSK-3β inhibitor, SB216763 (10 µM), pAPP was significantly reduced (-43%; P<0.001) in neurons challenged with HSV-1 for 10-60 min. Similar inhibition (-30%; P<0.001) was observed at 18-24 hours post infection (hpi). These data suggest that both virus binding to plasma membrane (occurring during 10-60 min exposures of neurons to HSV-1) and intracellular viral replication (i.e., 18-24 hpi) cause GSK-3β activation triggering APP phosphorylation. The latter is believed to be a key event in APP processing and Aβ production. Indeed, we found that the GSK-3β inhibitor, SB216763, also significantly reduced (-22%; P<0.001) the accumulation of Aβ in HSV-1-infected neurons at 24 hpi. Activation of GSK-3β may also inhibit the transcriptional program mediated by the cAMP response element-binding protein (CREB). We then checked whether CREB activity was altered in HSV-1-infected neurons. We found that HSV-1 inhibited CREB by reducing its phosphorylation at Ser133 and increasing that at Ser129. These effects were reversed by SB216763 thus showing their dependence on GSK-3β activation. Given that both CREB inhibition and Aβ may affect the synaptic transmission machinery, we investigated the effects of HSV-1 on the expression of the presynaptic proteins synapsin and synaptophysin. The expression of both proteins was significantly reduced in infected neurons (-58%; P<0.01). This reduction depended on both GSK-3β activation and Aβ buildup in neurons given that it was counteracted by cell treatments with either SB216763 or an antibody raised against Aβ (4G8).
Taken together, these findings suggest that HSV-1 alters the expression of synaptic proteins via activation of GSK-3β and intraneuronal accumulation of Aβ. These data further support the view that recurrent HSV-1 infections spreading to the CNS may contribute to the pathophysiology of Alzheimer’s disease.
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