Abnormally high levels of amyloid-β peptide (Aβ) are likely to play a major role in Alzheimer’s disease (AD) pathogenesis. However, Aβ tailored therapies have so far failed to enter the market. One possible reason for the failure could be related to the fact that the peptide is viewed exclusively as a culprit within the intricate pathophysiology of the disease. In fact, Aβ is normally produced and released in the brain during neuronal activity. Moreover, it is needed for memory and its cellular surrogate long-term potentiation (LTP), and has a positive modulatory effect on these phenomena when it is administered at low picomolar concentrations resembling the physiological content of the peptide in the brain. To better understand why and how a protein involved in synaptic plasticity in the healthy brain, at some point, accumulates leading to AD, we have sought to deepen the study of Aβ in physiological conditions. Specifically, we have investigated the relationship between Aβ and events normally occurring at the pre- and post-synaptic level. We have found that the Aβ-induced enhancement of plasticity and memory is associated with changes in presynaptic mechanisms such as paired pulse facilitation and post-tetanic potentiation, suggesting an increase of neurotransmitter release. Consistent with these presynaptic changes, Aβ-induced potentiation correlated with an increased expression of the vesicular glutamate transporter VGLUT1. The effects of picomolar concentrations of Aβ were not limited to the presynaptic side, as increased expression of the postsynaptic marker PSD-95 also correlated with Aβ-induced potentiation. Intriguingly, Aβ was able to convert protein synthesis independent early-LTP into protein synthesis dependent late-LTP. Taken all together, these findings led us to hypothesize that the nitric oxide (NO)/cGMP/cGMP-dependent protein kinase (cGK)/CREB cascade that is known to play a key role in pre- and post-synaptic mechanisms of plasticity and in the conversion from early- to late-LTP, might be involved in Aβ-mediated synaptic effects in physiological conditions. We have found that an increase of cGMP levels through the phosphodiesterase 5 inhibitor (PDE5-I) vardenafil increases Aβ levels both in N2A cells and hippocampal slices. OptiCAB assay allowed us to determine that the increase of cGMP stimulates Aβ production by increasing the interaction between amyloid precursor protein (APP) and the β-site APP cleaving enzyme 1 (BACE1). In addition, electrophysiological and behavioral studies have shown that blocking Aβ function (by anti-murine Aβ antibodies or APP knock-out mice) prevents the cGMP-dependent enhancement of LTP and memory, suggesting that cGMP acts through Aβ to boost LTP and memory. However, the role of the NO cascade in the physiological effect of Aβ is not confined to the upstream regulation of Aβ production, but it is also effective at the downstream level where it modulates Aβ function. Indeed, the Aβ-induced enhancement of LTP was blocked by the NO-synthase inhibitor L-NAME, or by the cGK inhibitor Rp-8-Br-cGMPS. Moreover, the enhancement correlated with an increased expression of neuronal NO synthase and phospho-CREB, suggesting that Aβ acts through the NO/cGMP/CREB signaling to positively modulate plasticity. In conclusion, we have found a tight relationship between cGMP and Aβ, demonstrating that cGMP acts both upstream of Aβ, by regulating its production, and downstream, by modulating its effects, thus suggesting the existence of a cGMP-Aβ-cGMP loop. This might be useful to better understand the mechanism of action of drugs increasing cGMP levels, such as PDE5-Is, that might exert their cognitive-enhancing effects via a positive modulation of Aβ in the brain both at the upstream and downstream levels. More importantly, these experiments stress the relevance of fully understanding the physiological role of Aβ to design effective and safe approaches to AD therapy, as Aβ lowering therapies might lead to memory worsening instead of ameliorating it.
cGMP and Aβ crosstalk: relevance of physiological synaptic mechanisms for Alzheimer’s disease
PUZZO, DANIELA
2016-01-01
Abstract
Abnormally high levels of amyloid-β peptide (Aβ) are likely to play a major role in Alzheimer’s disease (AD) pathogenesis. However, Aβ tailored therapies have so far failed to enter the market. One possible reason for the failure could be related to the fact that the peptide is viewed exclusively as a culprit within the intricate pathophysiology of the disease. In fact, Aβ is normally produced and released in the brain during neuronal activity. Moreover, it is needed for memory and its cellular surrogate long-term potentiation (LTP), and has a positive modulatory effect on these phenomena when it is administered at low picomolar concentrations resembling the physiological content of the peptide in the brain. To better understand why and how a protein involved in synaptic plasticity in the healthy brain, at some point, accumulates leading to AD, we have sought to deepen the study of Aβ in physiological conditions. Specifically, we have investigated the relationship between Aβ and events normally occurring at the pre- and post-synaptic level. We have found that the Aβ-induced enhancement of plasticity and memory is associated with changes in presynaptic mechanisms such as paired pulse facilitation and post-tetanic potentiation, suggesting an increase of neurotransmitter release. Consistent with these presynaptic changes, Aβ-induced potentiation correlated with an increased expression of the vesicular glutamate transporter VGLUT1. The effects of picomolar concentrations of Aβ were not limited to the presynaptic side, as increased expression of the postsynaptic marker PSD-95 also correlated with Aβ-induced potentiation. Intriguingly, Aβ was able to convert protein synthesis independent early-LTP into protein synthesis dependent late-LTP. Taken all together, these findings led us to hypothesize that the nitric oxide (NO)/cGMP/cGMP-dependent protein kinase (cGK)/CREB cascade that is known to play a key role in pre- and post-synaptic mechanisms of plasticity and in the conversion from early- to late-LTP, might be involved in Aβ-mediated synaptic effects in physiological conditions. We have found that an increase of cGMP levels through the phosphodiesterase 5 inhibitor (PDE5-I) vardenafil increases Aβ levels both in N2A cells and hippocampal slices. OptiCAB assay allowed us to determine that the increase of cGMP stimulates Aβ production by increasing the interaction between amyloid precursor protein (APP) and the β-site APP cleaving enzyme 1 (BACE1). In addition, electrophysiological and behavioral studies have shown that blocking Aβ function (by anti-murine Aβ antibodies or APP knock-out mice) prevents the cGMP-dependent enhancement of LTP and memory, suggesting that cGMP acts through Aβ to boost LTP and memory. However, the role of the NO cascade in the physiological effect of Aβ is not confined to the upstream regulation of Aβ production, but it is also effective at the downstream level where it modulates Aβ function. Indeed, the Aβ-induced enhancement of LTP was blocked by the NO-synthase inhibitor L-NAME, or by the cGK inhibitor Rp-8-Br-cGMPS. Moreover, the enhancement correlated with an increased expression of neuronal NO synthase and phospho-CREB, suggesting that Aβ acts through the NO/cGMP/CREB signaling to positively modulate plasticity. In conclusion, we have found a tight relationship between cGMP and Aβ, demonstrating that cGMP acts both upstream of Aβ, by regulating its production, and downstream, by modulating its effects, thus suggesting the existence of a cGMP-Aβ-cGMP loop. This might be useful to better understand the mechanism of action of drugs increasing cGMP levels, such as PDE5-Is, that might exert their cognitive-enhancing effects via a positive modulation of Aβ in the brain both at the upstream and downstream levels. More importantly, these experiments stress the relevance of fully understanding the physiological role of Aβ to design effective and safe approaches to AD therapy, as Aβ lowering therapies might lead to memory worsening instead of ameliorating it.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
