26 June 2009. Giving Alzheimer mice a three-week regimen of Viagra (sildenafil) sharpens their memories and lessens the load of amyloid-β (Aβ) in their brains, according to a study published this week. Sildenafil and similar medicines work by inhibiting phosphodiesterase 5 (PDE5) and elevating the signaling molecule cGMP, but they have been studied almost exclusively in the peripheral vascular system. The work from Ottavio Arancio and colleagues at Columbia University, New York, shows that inhibiting PDE5 in the brain leads to long-lasting improvements in synaptic function, behavior, and amyloid pathology.
The data are not an endorsement of Viagra for dementia, the authors stress. That will require new drugs with better brain penetration and higher selectivity for PDE5 over other PDEs. “Our findings should be interpreted as proof of concept in favor of developing new PDE5 inhibitors that are optimized for the CNS,” they write.
The results appear in the June 24 issue of the Journal of Neuroscience, which also contains a paper on an immunization study that was presented at last year’s Society for Neuroscience meeting (see ARF related news story). Carol Colton and Donna Wilcock, Duke University, Durham, North Carolina, had previously shown the protective effects of nitric oxide (an important elevator of cGMP) in a mouse model of AD, where knockout of nitric oxide synthase causes a more extensive spectrum of pathology compared to other AD mouse models. The NOS knockout mice that also express human APP with the Swedish mutation show enhanced amyloid accumulation, but also tau pathology derived from endogenous normal tau, and, unlike other models, neuron death (see ARF related news story on Wilcock et al., 2008). Their latest paper reports the effects of immunizing the mice with Aβ. The results are encouraging: In addition to a reduction in amyloid, immunization also led to a loss of tau pathology, ameliorated neuronal death, and improved behavioral measures in the mice. Their work suggests that removal of Aβ could be sufficient to improve tau and other downstream events. There was a note of caution, though. Immunization also increased microhemorrhage in the mice.
The idea to test sildenafil and other PDE5 inhibitors in AD mice comes from studies linking elevation of cGMP to phosphorylation of the transcription factor cAMP-responsive element binding factor (CREB). Aβ decreases CREB phosphorylation, but that can be prevented by cGMP analogs or NO donors, which also raise cGMP, and are able to protect Aβ-stressed neurons (Puzzo et al., 2005).
In the study, first author Daniela Puzzo and colleagues tested the effects of sildenafil and an even more specific PDE5 inhibitor, tadalafil, on neuronal function by measuring long-term potentiation in hippocampal slices from APP/PS1 mice. The slices exhibit defective LTP in vitro, which was restored by the inhibitors. The researchers then chose to test sildenafil in vivo, because it can cross the blood-brain barrier. A three-week daily dosing of sildenafil improved the animals’ performance in a test of hippocampal-dependent contextual fear learning, but not hippocampal-independent cued fear learning, and also boosted performance in a test of spatial working memory. To confirm that the effect stemmed from PDE5 inhibition in brain and not in the peripheral vasculature, the researchers also tested tadalafil in vivo. That compound, which cannot cross the blood-brain barrier, did not have any effects on memory despite restoring LTP in hippocampal slices.
A most interesting result came when the investigators looked at the time course of sildenafil action. They found that treating young mice for three weeks resulted in improvements in behavior that lasted for 9-12 weeks after the drug was discontinued. The same long-lasting effect was seen for LTP measured in slices from treated mice. In older mice, the situation was a little different. While LTP was improved by sildenafil, defects in basal neurotransmission were not. This suggests that once synapse damage occurs, PDE5 inhibition can correct synaptic plasticity, but not basal synaptic function.
The actions of sildenafil on PDE5 inhibition are short-lived, and could not account for the long-term effects on LTP or behavior. To explain that, the investigators looked at the state of CREB phosphorylation, and found that treatment caused a sustained restoration of tetanus-induced increases in CREB phosphorylation, a response that is defective in untreated APP/PS1 mice. Since Aβ itself inhibits CREB phosphorylation, they measured Aβ levels directly. Sildenafil treatment modestly but significantly reduced Aβ levels even 12 weeks after dosing.
“These findings support a model in which PDE5 inhibitors counteract the deficit in CREB phosphorylation by Aβ, not only immediately but also for a prolonged period of time through regulation of transcription of genes controlling Aβ synthesis/degradation,” the authors write. This result is in agreement with the work of Colton and colleagues, who have shown that Aβ levels are increased in their NO knockout/AD mice.
The data point to sildenafil acting largely through brain PDE5, but it is possible that it could also have vascular effects. Aβ present in the bloodstream promotes vasoconstriction, and high blood pressure is a risk factor for AD. Perhaps, the authors speculate, PDE5 inhibition might counteract not only the CNS issues, but also some of the vascular symptoms of AD.—Pat McCaffrey.
Puzzo D, Staniszewski A, XianDeng S, Privitera L, Leznik E, Liu S, Zhang H, Feng Y, Palmeri A, Landry DW, Arancio O. Phosphodiesterase 5 inhibition improves synaptic function, memory and amyloid-b load in Alzheimer's disease mouse model. J. Neurosci. 2009 June 24; 25:8075-8086. Abstract
Wilcock DM, Nastaran G, Van Nostrand WE, Davis J, Vitek MP, Colton CA. Amyloid reduction by amyloid-beta vaccination also reduces mouse tau pathology and protects from neuron loss in two mouse models of Alzheimer’s disease. 2009 June 24; 25:7957-7965. Abstract