Protein phosphatase-1(PP1)plays a critical role in synaptic plasticity and memory formation.PP1 activity is required for the induction of long term depression(LTD)and PP1 inhibition by endogenous signaling pathways is required for the induction of long term potentiation(LTP).In the nucleus,PP1 is a major phosphatase for CREB at Ser133,phosphorylation of which activates CREB-mediated gene expression.Active PP1 suppresses memory formation via its effect on CREB mediated gene expression and LTP/LTD induction.Inhibitor-1(I-1)was originally suggested to be the critical regulator of PP1 signaling in LTD induction.However,enigmatically,LTD was found to be normal in I-1 KO mice.Thus the mechanisms by which PP1 function is controlled in both plasticity and cognition has remained unclear.Our studies indicate that knocking down of inhibitor-2(I-2),but not of I-1,leads to a change of PP1 activity in hippocampal and cortical neurons,suggesting that I-2 is a major PP1 regulatory protein in these brain regions.We found that I-2 phosphorylation at Ser43 is bi-directionally regulated by prolonged increase or decrease of neuronal activity,and that this regulation is mediated by L-type calcium channels,but not by NMDA receptors.The regulation of I-2 phosphorylation at Ser43 is critical for homeostatic synaptic plasticity.On the other hand,NMDA receptor activation decreases I-2 phosphorylation at Thr72 and increases I-2-PP1 complex formation.Moreover,we have found that I-2 is necessary for the induction of NMDA receptor dependent LTD.Furthermore,we found that I-2 constrains memory formation based on studies with rats injected with lentiviruses expressing I-2 ShRNA(ShI-2)or with I-2+/-mice generated in our laboratory(I-2-/-mice die as embryos).Based on existing knowledge that PP1 constrains learning and memory,our data suggest that I-2 is a positive regulator of PP1 and a novel memory suppressor.Lastly,we found that CREB phosphorylation at Ser133 is increased in I-2 KD neurons,suggesting that CREB-mediated gene expression contributes to I-2 function in cognition.Our studies thus demonstrate that I-2 is a positive in vivo regulator of PP1 function in synaptic plasticity and memory formation.