Neuroprotective Mechanism(s) of Purines

Stroke caused by cerebral ischemia is the third leading cause of death and the leading cause of neurological disability. During hypoxia ATP production is diminuished and the energy status of neurons and glia is compromised, ionic gradients dissipate, and cell membranes depolarize. Concomitant with energy depletion is the formation of the purine nucleoside adenosine, a powerful endogenous neuroprotectant which e.g. may decrease neuronal metabolism and increase cerebral blood flow. In animal models, an impressive reduction of neuronal damage and mortality hardly attained by any other drug has been observed. Adenosine acting through A1 receptors has neuroprotective effects probably by inhibiting excitatory synaptic neurotransmission in the brain during hypoxia. In contrast, activation of A2A receptors may enhance neuronal damage. Recent results however, indicated that hypoxia-induced membrane responses of PC12 cells are likely to be mediated via activation of the A2A receptor. Adenosine A3 agonists are cytoprotective, but at higher concentrations may also enhance ischemia- induced neuronal damage. Inosine, an adenosine derivative and guanosine were shown to preserve glial cell viability. Many aspects of the mechanisms involved in purine-mediated protection remain unclear. The purpose of this study is to investigate the neuroprotective mechanisms of adenosine, guanosine and inosine in `ischemic` neuronal cell cultures and in an in vivo model for ischemia/reperfusion. Specific aims are: 1) the evaluation of the influence of purines on hypoxia mediated apoptosis/necrosis of a neuronal cell line and on primary cerebellar granule cells, 2) the study of the role of adenosine receptors, 3) the analysis of the effect of purines on signal transduction in neuronal cell lines, 4) the identification of physiological effector molecules of purine dependent signal transduction pathways by mRNA profiling and as a lon-term goal 5) the study of purine- mediated neuroprotection in an in-vivo model for ischemic/reperfusion. The solemn understanding of purine-mediated cytoprotection should further add to the development of purine- based therapeutic approaches to stroke and related disorders.

Stroke caused by cerebral ischemia is the third leading cause of death and the leading cause of neurological disability. During hypoxia the cerebral blood flow is reduced which leads to neuronal cell death by necrosis (in the core of the infarct) or apoptosis in the periphery. Concomitant with energy depletion is the formation of the purine nucleoside adenosine, a powerful endogenous neuroprotectant, which e.g. may decrease neuronal metabolism and increase cerebral blood flow. The purpose of this FWF funded study was to investigate the neuroprotective mechanisms of adenosine and its relatives, guanosine and inosine, in `ischemic` neuronal cells. Results showed that purine nucleosides may indeed rescue viability and support neurite outgrowth of neuronal cells, which were subjected to hypoxia. Additionally, data revealed the vital role of certain survival kinases, such as the mitogen activated protein kinase, for the purine nucleoside-mediated protection. In future studies, special emphasis will therefore be put on the investigation of these signaling pathways. In the long run results of our work should support current efforts to develop therapies for pathologies involving ischemia/reperfusion injuries and neurodegenerative diseases.