Certain Chemicals Produced By The Enzyme Cox-2 Protect The Brain
A study by Johns Hopkins researchers has revealed that stimulating brain cell receptors for certain hormone-like chemicals in brain cells called prostaglandins can protect the cells from amyloid ß-peptide 42 (Aß1-42), a compound that has been linked to brain cell death and Alzheimer's disease (AD).
Prostaglandin E2 (PGE2) is produced via the action of the COX-2 enzyme, which can contribute to brain injury. In spite of the negative effects of COX-2, ongoing studies have shown that PGE2 can actually provide some protection against brain cell death by binding to various PGE2 receptors.
Prostaglandins are a class of compounds that act like hormones by binding to specific receptors. Their a number of functions include constricting and relaxing blood vessels, controlling clotting, causing pain, and both increasing and decreasing inflammation.
Because neuroinflammation is thought to play a role in the development of AD, PGE2 was a logical place to look for clues to AD toxicity and brain cell death, according to co-lead scientist Sylvain Dore, Ph.D., an associate professor of anesthesiology and critical care medicine and neuroscience at The Johns Hopkins University School of Medicine.
Eventhough it was already known that PGE2 can offer some protection against neurotoxicity, Dore's study shows that this protection is linked to stimulation of receptors EP2 and EP4. This stimulation results in a cascade of events inside brain cells that produces cyclic-AMP (cAMP), a molecule that protects brain cells by reducing the toxic effects of Aß1-42.
Dore speculates that the presence of Aß1-42 in neuritic plaque, a waxy translucent substance consisting of protein and other materials, a hallmark in the brains of AD patients, may cause cellular death by self-assembling into long protein filaments that are toxic to neurons.
It's also possible, Dore said, that prostaglandin protection works by modifying the link between Aß1-42 and the overproduction of free radicals. Free radicals are highly reactive chemicals that oxidize other molecules and at high concentrations lead to cell death. Free radicals are associated with neuronal loss observed in AD.
"The development and testing of molecules that can enhance PGE2 receptor activity, and further research into how these receptors increase cAMP concentrations and improve protection could lead to successful new treatments," Dore said.
In the study, published in the European Journal of Neuroscience, Dore and scientists focused on four specific PGE2 receptors, EP1-4, in cortical neuronal cells cultured from postnatal mice.
To establish Aß1-42-induced neurotoxicity, Dore and his team incubated these neurons with freshly dissolved Aß1-42 protein for 48 hours. The analysis of the cells showed that Aß1-42 resulted in a net increase in neuronal cell death compared to control cells that did not receive the peptide.
To investigate the effect of PGE2 on Aß1-42 toxicity, neurons were co-treated with Aß1-42 and different concentrations of PGE2. Results showed that PGE2 significantly increased cell survival compared to cultures that received only Aß1-42.
To determine which of the four PGE2 receptors was responsible in the protection against Aß1-42 toxicity, Dore's group conducted three separate experiments. In the first they co-treated neurons with Aß1-42 and the EP2 agonist butaprost. An agonist is a drug that mimics the action of a natural substance and binds to that substance's receptor. Results showed that the stimulation of EP2 receptors offered significant protection against Aß1-42 neurotoxicity.
They also co-treated neurons with Aß1-42 and the EP4/EP3 agonist, OHPGE1, and received similar results.
Conversely, co-treatment of the cells with the EP3/EP1 agonist, sulprostone, and Aß1-42 exhibited no significant protection.
Dore's group concluded that the protective effects against Aß1-42 neurotoxicity are specific to PGE2 receptors EP2 and EP4.
The scientists next pursued changes in cAMP levels as a potential underlying cellular mechanism in the protective actions of EP2 and EP4 agonists. They treated neurons with PGE2, butaprost or OHPGE1 for 15 minutes and measured the cAMP concentration inside the cells. Results showed that brief exposure of neurons to PGE2 almost tripled cAMP levels, and exposure to butaprost or OHPGE1 almost doubled them.
Subsequently, to address whether PGE2-mediated neuroprotection involves cAMP, Dore and his group measured neuron toxicity of Aß1-42 in the absence or presence of cAMP. Treatment with cAMP significantly enhanced cell health after Aß1-42 exposure indicating that stimulation of PGE2 receptors EP2 and EP4 generates a cascade of events that increases cAMP concentrations and, in turn, reduces Aß1-42 neurotoxicity.
"Due to the established link between Aß1-42 and Alzheimer's disease, this discovery could lead to better drug therapies for treating this disease," Dore said.
Additional scientists in this study include co-lead author and former Hopkins scientist Valentina Echeverria, Ph.D., and Andrew Clerman, B.S., a graduate student in the Department of Anesthesiology and Critical Care Medicine.
This study was funded by grants from the National Institutes of Health.