Adenosine A1 receptor protein levels and activity is increased in the cerebral cortex in Creutzfeldt-Jakob disease and in bovine spongiform encephalopathy-infected bovine-PrP mice

Abstract

Prion diseases are characterized by neuronal loss, astrocytic gliosis, spongiform change, and abnormal protease-resistant prion protein (PrP) deposition. Creutzfeldt-Jakob disease (CJD) is the most prevalent human prion disease, whereas scrapie and bovine spongiform encephalopathy (BSE) are the most common animal prion diseases. Several candidates have been proposed as mediators of degeneration in prion diseases, one of them glutamate. Recent studies have shown reduced metabotropic glutamate receptor/phospholipase C signaling in the cerebral cortex in CJD, suggesting that this important neuromodulator and neuroprotector pathway is attenuated in CJD. Adenosine is involved in the regulation of different metabolic processes under physiological and pathologic conditions. Adenosine function is mediated by adenosine receptors, which are categorized into 4 types: A1, A2A, A2B, and A3. A1Rs are G-protein-coupled receptors that induce the inhibition of adenylyl cyclase activity. The most dramatic inhibitory actions of adenosine receptors are on the glutamatergic system. For these reasons, we examined the levels of A1Rs in the frontal cortex of 12 patients with CJD and 6 age-matched controls and in BSE-infected bovine-PrP transgenic mice (BoPrP-Tg110 mice) at different postincubation times to address modifications in A1Rs with disease progression. A significant increase in the protein levels of A1Rs was found in the cerebral cortex in CJD and in the murine BSE model at advanced stages of the disease and coincidental with the appearance of PrP expression. In addition, the activity of A1Rs was analyzed by in vitro assays with isolated membranes of the frontal cortex in CJD. Increased activity of the receptor, as revealed by the decreased forskolin-stimulated cAMP production in response to the A1R agonists cyclohexyl adenosine and cyclopentyl adenosine, was observed in CJD cases when compared with controls. Finally, mRNA A1R levels were similar in CJD and control cases, thus suggesting abnormal A1R turnover or dysregulation of raft-associated signaling pathways in CJD. These results show, for the first time, sensitization of A1Rs in prion diseases.