Regulation of proteasome activity by P2Y receptor underlies the neuroprotective effects of extracellular nucleotides.

The Ubiquitin-Proteasome System (UPS) is essential for the regulation of the cellular proteostasis. Indeed, it has been postulated that an UPS dysregulation is the common mechanism that underlies several neurological disorders. Considering that extracellular nucleotides, through their selective P2Y receptor (P2YR), play a neuroprotective role in various neurological disorders that course with an UPS impairment, we wonder if this neuroprotective capacity resulted from their ability to modulate the UPS. Using a cellular model expressing two different UPS reporters, we found that the stimulation of P2YR by its selective agonist UpU induced a significant reduction of UPS reporter levels. This reduction was due to an increase in two of the three peptidase proteasome activities, chymotrypsin and postglutamyl, caused by an increased expression of proteasome constitutive catalytic subunits β1 and β5. The intracellular signaling pathway involved required the activation of IP/MEK1/2/ERK but was independent of PKC or PKA. Interestingly, the P2YR activation was able to revert both UPS-reporter accumulation and the cell death induced by a prolonged inhibition of UPS. Finally, we also observed that intracerebroventricular administration of UpU induced a significant increase both of chymotrypsin and postglutamyl activities as well as an increased expression of proteasome subunits β1 and β5 in the hippocampus of wild-type mice, but not in P2YR KO mice. All these results strongly suggest that the capacity to modulate the UPS activity via P2YR is the molecular mechanism which is how the nucleotides play a neuroprotective role in neurological disorders.