Protective mechanisms of adenosine in neurons and glial cells.
As illustrated in Figure 1, a disturbance of the intracellular Ca2+ homeostasis is thought to be a common pathogenic factor for the generation of secondary nerve cell damage that develops after brain trauma or stroke or during the course of neurodegenerative diseases. A neuronal Ca2+ overload which may result from an excessive glutamate-evoked membrane depolarization and consecutive Ca2+ influx as well as from an activation of metabotropic receptors and consecutive intracellular Ca2+ mobilization is known to have direct toxic effects on the cytoskeleton and the cell metabolism of neurons. In addition, a Ca(2+)-dependent activation of glial cells along with the loss of physiologically required mature astrocyte functions and with the acquisition of potentially neurotoxic microglial properties, has more recently been recognized as an additive pathogenic factor. This may provide an effective target for pharmacological interference. Specifically, the reinforcement of an endogenous homeostatic regulator, which obtained its sophisticated know-how during evolution, may provide a neuroprotective therapy which can handle the complexity of the pathological process with a minor risk of pharmacological side effects. Adenosine is such an ancient molecular signal that acts on both neurons and glial cells. In neurons, adenosine activates K+ and Cl- conductances, which limits synaptically evoked depolarization, thus counteracting the Ca2+ influx through voltage-dependent and NMDA receptor-operated ion channels. This A1 receptor-mediated effect seems to be the major action by which adenosine adds directly to the protection of neurons against Ca(2+)-dependent damage. In glial cells, the prevalent effect of adenosine is its regulatory influence on the Ca2+ and cAMP-dependent molecular signaling that determines the cellular proliferation rate, the differentiation state and related functions. When mimicking the activation of metabotropic glutamate receptors in cultures of immature rat astrocytes, which largely resemble pathologically activated astrocytes, a transient Ca2+ mobilization was initiated by adenosine. This A1 receptor-mediated Ca2+ signal caused a prolonged potentiation of the A2 receptor-mediated intracellular cAMP rise. An experimentally sustained enhancement of the cAMP signaling initiated the differentiation of cultured astrocytes and the new expression of K+ and Cl- channels which are required for the physiological astrocyte function to maintain the extracellular ion homeostasis. Evidence is accumulating that a strengthening of the cAMP signaling, which can be achieved by adenosine agonists and also by the pharmacon propentofylline (an adenosine uptake blocker and phosphodiesterase inhibitor), stimulates the mRNA production of neurotrophic factors in astrocytes. In cultured microglial cells, several days' treatment with adenosine agonists or propentofylline markedly inhibited their proliferation rate, the in vitro spontaneously occurring transformation into macrophages and their particularly high formation of free oxygen radicals. Adenosine agonists also depressed the release of the potentially toxic cytokine TNF alpha and induced programmed cell death in immunologically activated microglial cells. We conclude that a pharmacological reinforcement of the endogenous cell modulator adenosine may provide neuroprotection by counteracting neuronal Ca2+ overload, by depressing potentially neurotoxic microglial functions and by regaining physiologically required properties of differentiated astrocytes. Further information about the influence of adenosine on the molecular signaling and on ischemic brain damage is given in Refs. 37 and 38, and about the implicated possible relevance for the treatment of stroke in Ref. 39.