ABSTRACT
Title
GPR17: a new receptor involved in the evolution, remodelling and repair of brain's ischemic lesions
Authors
L. Sironi1, P. Gelosa1, D. Lecca1, U. Guerrini1, E. Tremoli1,3, M. Cimino*2, and M.P. Abbracchio*1
1Dept. of Pharmacological Sciences, University of Milan, Milano, Italy
2Dept. of Biomolecular Sciences, University of Urbino “Carlo Bo”, Italy
3Centro Cardiologico Monzino, IRCCS, Milano, Italy
* MC e MPA contributed equally to this work
1Dept. of Pharmacological Sciences, University of Milan, Milano, Italy
2Dept. of Biomolecular Sciences, University of Urbino “Carlo Bo”, Italy
3Centro Cardiologico Monzino, IRCCS, Milano, Italy
* MC e MPA contributed equally to this work
Abstract
Cerebral ischemia is the third most common cause of death in Western countries and a leading cause of permanent disability, but despite the intensive basic and clinical research it still remains an unmet medical need awaiting new drugs able to counteract the associated neurodegenerating effects. It is also currently acknowledged that, due to difficulties in diagnostic time, an ideal intervention to stroke should comprise both neuroprotective and neurorestorative approaches aimed at implementing the local post-injury repair mechanisms.
A new P2Y-like G-protein-coupled receptor, named GPR17, activated by both uracil nucleotides and cysteinyl- leukotrienes (cysLTs) has been cloned and found to be highly expressed in brain. Data obtained in rodents using an established focal ischemia model, obtained by the occlusion of middle cerebral artery (MCAo), suggest that GPR17 may act as a “sensor” of danger, by both activating local tissue response upon acute damage and by triggering remodelling and repair in the subsequent post-ischemic phase. The role of GPR17 in ischemia evolution, remodelling and repair has been evaluated by both immunohistochemical and Magnetic Resonance Imaging (MRI) approaches. In a first set of experiments, we showed that, in the intact mouse brain, GPR17 is normally expressed at low levels in neurons and prominently in a subset of NG2+/Olig2+ precursor cells (OPCs) dispersed in brain’s parenchyma, that never proliferated under control conditions, suggesting that they are quiescent pre-oligodendrocytes. Induction of ischemic injury by MCAo induced profound spatiotemporal-dependent changes of GPR17. In the lesioned area, an early and transient up-regulation of GPR17 was recorded in dying neurons expressing the cellular stress marker heat shock protein 70 (HSP70), a chaperon protein up-regulated in neuron upon damage, suggesting contribution to cell death. MRI in living mice indeed confirmed that, at this early ischemic stage, the pharmacological or acute biotechnological knock down of GPR17 markedly prevented brain infarct evolution [1, 2]. At later times after ischemia (48 hours), GPR17 immuno-labelling appeared at the boundaries of the lesioned area associated to cells positive also for IB4marker, suggesting that they are on microglia/macrophages infiltrating the lesioned area. Starting from 72 hours after MCAo, proliferating GPR17+ OPCs were consistently found in the striatum as well as in the region surrounding the ischemic area. Some of these proliferating precursor cells also positively stained for APC, a marker of mature oligodendroglia, suggesting that GPR17+ OPCs are activated by the ischemic insult and induced to differentiate to mature oligodendrocytes, in an attempt to re-myelinate lesions. The latter hypothesis is reinforced by data obtained with MRI techniques showing, starting from 3-4 weeks after MCAo, an increase in fractional anisotropy (FA) ratio, an index reflecting the microstructure and integrity of the white matter, in the boundaries of the ischemic areas. Interestingly, in the areas characterized by FA ratio increase, the immunohistochemical analysis detected the presence of GPR17+ cells as well as of CNPase+ and MBP+ mature oligodendrocytes. Thus, GRP17 may be involved in the reorganization of damaged myelin after an ischemic insult. To support this hypothesis, GPR17 agonists promoted and GPR17 antagonists delayed the in vitromaturation of OPCs to mature myelinating oligodendrocytes [3]. These results globally suggest GPR17 as an interesting new target not only for stroke, which is nevertheless characterized by marked oligodendrocyte dysfunction and myelin deterioration, but also for human neurodegenerative diseases characterized by demyelination.
[1] Ciana P. et al. (2006) EMBO J25, 4615-4627.
[2] Lecca D. et al. (2008) PLoS One10, e3579.
[3] Fumagalli M et al (2011) J Biol Chem, published on line on Jan 5.
A new P2Y-like G-protein-coupled receptor, named GPR17, activated by both uracil nucleotides and cysteinyl- leukotrienes (cysLTs) has been cloned and found to be highly expressed in brain. Data obtained in rodents using an established focal ischemia model, obtained by the occlusion of middle cerebral artery (MCAo), suggest that GPR17 may act as a “sensor” of danger, by both activating local tissue response upon acute damage and by triggering remodelling and repair in the subsequent post-ischemic phase. The role of GPR17 in ischemia evolution, remodelling and repair has been evaluated by both immunohistochemical and Magnetic Resonance Imaging (MRI) approaches. In a first set of experiments, we showed that, in the intact mouse brain, GPR17 is normally expressed at low levels in neurons and prominently in a subset of NG2+/Olig2+ precursor cells (OPCs) dispersed in brain’s parenchyma, that never proliferated under control conditions, suggesting that they are quiescent pre-oligodendrocytes. Induction of ischemic injury by MCAo induced profound spatiotemporal-dependent changes of GPR17. In the lesioned area, an early and transient up-regulation of GPR17 was recorded in dying neurons expressing the cellular stress marker heat shock protein 70 (HSP70), a chaperon protein up-regulated in neuron upon damage, suggesting contribution to cell death. MRI in living mice indeed confirmed that, at this early ischemic stage, the pharmacological or acute biotechnological knock down of GPR17 markedly prevented brain infarct evolution [1, 2]. At later times after ischemia (48 hours), GPR17 immuno-labelling appeared at the boundaries of the lesioned area associated to cells positive also for IB4marker, suggesting that they are on microglia/macrophages infiltrating the lesioned area. Starting from 72 hours after MCAo, proliferating GPR17+ OPCs were consistently found in the striatum as well as in the region surrounding the ischemic area. Some of these proliferating precursor cells also positively stained for APC, a marker of mature oligodendroglia, suggesting that GPR17+ OPCs are activated by the ischemic insult and induced to differentiate to mature oligodendrocytes, in an attempt to re-myelinate lesions. The latter hypothesis is reinforced by data obtained with MRI techniques showing, starting from 3-4 weeks after MCAo, an increase in fractional anisotropy (FA) ratio, an index reflecting the microstructure and integrity of the white matter, in the boundaries of the ischemic areas. Interestingly, in the areas characterized by FA ratio increase, the immunohistochemical analysis detected the presence of GPR17+ cells as well as of CNPase+ and MBP+ mature oligodendrocytes. Thus, GRP17 may be involved in the reorganization of damaged myelin after an ischemic insult. To support this hypothesis, GPR17 agonists promoted and GPR17 antagonists delayed the in vitromaturation of OPCs to mature myelinating oligodendrocytes [3]. These results globally suggest GPR17 as an interesting new target not only for stroke, which is nevertheless characterized by marked oligodendrocyte dysfunction and myelin deterioration, but also for human neurodegenerative diseases characterized by demyelination.
[1] Ciana P. et al. (2006) EMBO J25, 4615-4627.
[2] Lecca D. et al. (2008) PLoS One10, e3579.
[3] Fumagalli M et al (2011) J Biol Chem, published on line on Jan 5.