[Show abstract][Hide abstract] ABSTRACT: The rate of ischemic brain injury varies with the brain region, requiring only hours in striatum but days in hippocampus. Such maturation implies the existence of endogenous neuroprotective mechanisms. Adenosine is an endogenous neuroprotectant regulated by adenosine kinase (ADK). To investigate, whether adenosine might play a role in protecting the hippocampus after focal ischemia, we subjected transgenic mice, which overexpress ADK in hippocampal neurons (Adk-tg mice) to transient middle cerebral artery occlusion (MCAO). Although the hippocampus of wild-type (wt) mice was consistently spared from injury after 60 mins of MCAO, hippocampal injury became evident in Adk-tg mice after only 15 mins of MCAO. To determine, whether downregulation of hippocampal ADK might qualify as candidate mechanism mediating endogenous neuroprotection, we evaluated ADK expression in wt mice after several periods of reperfusion after 15 or 60 mins of MCAO. After 60 mins of MCAO, hippocampal ADK was significantly reduced in both hemispheres after 1, 3, and 24 h of reperfusion. Reduction of ADK-immunoreactivity corresponded to a 2.2-fold increase in hippocampal adenosine at 3 h of reperfusion. Remarkably, a significant reduction of ADK immunoreactivity was also found in the ipsilateral (stroked) hippocampus after 15 mins of MCAO and 3 h of reperfusion. Thus, transient downregulation of hippocampal ADK after stroke might be a protective mechanism during maturation hippocampal cell loss.
[Show abstract][Hide abstract] ABSTRACT: Protective mechanisms of the brain may reduce the extent of injury after focal cerebral ischemia. Here, we explored in a mouse model of focal cerebral ischemia potential synergistic neuroprotective effects of two mediators of neuroprotection: (i) neuronal or glial precursor cells and (ii) the inhibitory neuromodulator adenosine. Embryonic stem (ES) cells, engineered to release adenosine by biallelic disruption of the adenosine kinase gene, and respective wild-type cells were induced to differentiate into either neural or glial precursor cells and were injected into the striatum of mice 1 week before middle cerebral artery occlusion. All stem cell-derived graft recipients were characterized by a significant reduction in infarct volume, an effect that was augmented by the release of adenosine. Neuroprotection was strongest in adenosine-releasing glial precursor cell recipients, which were characterized by an 85% reduction of the infarct area. Graft-mediated neuroprotection correlated with a significant improvement of general and focal neurologic scores. Histologic analysis before and after ischemia revealed clusters of implanted cells within the striatum of all treated mice. We conclude that ES cell derived adenosine-releasing brain implants provide neuroprotection by synergism of endogenous precursor cell-mediated effects and paracrine adenosine release.
[Show abstract][Hide abstract] ABSTRACT: Adenosine is a potent modulator of excitatory neurotransmission, especially in seizure-prone regions such as the hippocampal formation. In adult brain ambient levels of adenosine are controlled by adenosine kinase (ADK), the major adenosine-metabolizing enzyme, expressed most strongly in astrocytes. Since ontogeny of the adenosine system is largely unknown, we investigated ADK expression and cellular localization during postnatal development of the mouse brain, using immunofluorescence staining with cell-type specific markers. At early postnatal stages ADK immunoreactivity was prominent in neurons, notably in cerebral cortex and hippocampus. Thereafter, as seen best in hippocampus, ADK gradually disappeared from neurons and appeared in newly developed nestin- and glial fibrillary acidic protein (GFAP)-positive astrocytes. Furthermore, the region-specific downregulation of neuronal ADK coincided with the onset of myelination, as visualized by myelin basic protein staining. After postnatal day 14 (P14), the transition from neuronal to astrocytic ADK expression was complete, except in a subset of neurons that retained ADK until adulthood in specific regions, such as striatum. Moreover, neuronal progenitors in the adult dentate gyrus lacked ADK. Finally, recordings of excitatory field potentials in acute slice preparations revealed a reduced adenosinergic inhibition in P14 hippocampus compared with adult. These findings suggest distinct roles for adenosine in the developing and adult brain. First, ADK expression in young neurons may provide a salvage pathway to utilize adenosine in nucleic acid synthesis, thus supporting differentiation and plasticity and influencing myelination; and second, adult ADK expression in astrocytes may offer a mechanism to regulate adenosine levels as a function of metabolic needs and synaptic activity, thus contributing to the differential resistance of young and adult animals to seizures.