P2Y12 but not P2Y13 Purinergic Receptor Controls Postnatal Rat Retinogenesis In Vivo

Adenine nucleotides through P2Y(1) receptor stimulation are known to control retinal progenitor cell (RPC) proliferation by modulating expression of the p57(KIP2), a cell cycle regulator. However, the role of Gi protein-coupled P2Y(12) and P2Y(13) receptors also activated by adenine nucleotides in RPC proliferation is still unknown. Gene expression of the purinergic P2Y(12) subtype was detected in rat retina during early postnatal days (P0 to P5), while expression levels of P2Y(13) were low. Immunohistochemistry assays performed with rat retina on P3 revealed P2Y(12) receptor expression in both Ki-67-positive cells in the neuroblastic layer and Ki-67-negative cells in the ganglion cell layer and inner nuclear layer. Nonetheless, P2Y(13) receptor expression could not be detected in any stratum of rat retina. Intravitreal injection of PSB 0739 or clopidogrel, both selective P2Y(12) receptor antagonists, increased by 20 and 15%, respectively, the number of Ki-67-positive cells following 24h of exposure. Moreover, the P2Y(12) receptor inhibition increased cyclin D1 and decreased p57(KIP2) expression. However, there were no changes in the S phase of the cell cycle (BrdU-positive cells) or in mitosis (phospho-histone-H3-positive cells). Interestingly, an increase in the number of cyclin D1/TUNEL-positive cells after treatment with PSB 0739 was observed. These data suggest that activation of P2Y(12) receptors is required for the successful exit of RPCs from cell cycle in the postnatal rat retina.