Role of PI3-Kinase regulatory subunit (p85) isoforms in synaptic plasticity

Abstract

Learning and memory are widely thought to rely on synaptic plasticity, that is the ability of neurons to undergo activity-dependent modifications in the strength of synaptic transmission. This phenomenon contributes to modify the existing neuronal connections by changing the postsynaptic number/function of neurotransmitter receptors or the amount/probability of presynaptic neurotransmitter release. These functional aspects of synaptic plasticity (functional synaptic plasticity) are often accompanied by structural changes in dendritic spines (structural synaptic plasticity). However, the underlying molecular mechanisms connecting functional and structural synaptic plasticity remain poorly defined. Class I Phosphatidylinositol-3-Kinases (PI3Ks) constitute a complex family of enzymes formed by a p110 catalytic (p110α, p110β, p110γ, p110δ) and a p85 regulatory (p85α, p55α, p50α, p55γ, p85β) subunit, with important roles in several processes including synaptic plasticity and cognitive function. Moreover, p85 subunits also exist in a free p110-independent state. Several studies have focused on the requirement of PI3K catalytic activity for synaptic plasticity. However, the differential interactions and contributions of the PI3K regulatory isoforms (free or bound to p110s) to the functional and structural aspects of synaptic plasticity are still unknown. To address this issue, we have carried out loss-of-function (RNA interference) of specific p85 isoforms in rat hippocampal slices. After specific knockdown of p85α or p85β, most PI3K complexes are detected as heterodimers of p110 interacting with the alternative p85 isoform. In this manner, we can assess the function of p110/p85α versus p110/p85β forms of PI3K, without altering global amount of p110 heterodimers. Our electrophysiology experiments with organotypic hippocampal slices demonstrate that neither p85α nor p85β are required for mGluR-dependent long-term depression (LTD). In contrast, p85α (and not p85β) is critically required for NMDAR-dependent longterm potentiation (LTP). In addition, using live-cell imaging experiments, we show that p85α is also required for cofilin recruitment into spines after LTP induction. Interestingly, removal of p85β enhances both processes (synaptic potentiation and cofilin recruitment), possibly because of the larger contribution of p110/p85α heterodimers under these conditions. Moreover, we have detected that the small GTPases Rac1 and Cdc42 interact preferentially with the BH domain of p85α, as compared to p85β. In agreement with this biochemical observation, p85α is required for Rac1 activation triggered by PI3K activity, and for actin polymerization in dendritic spines after LTP induction. Thus, we propose a mechanism for p85α to modulate actin dynamics during spine structural plasticity through its specific interaction with Rac1. The results in this PhD thesis show that the regulatory subunits provide functional specializations to PI3Ks, with a preponderant role of p85α in synaptic plasticity, particularly for actin dynamics during long-term synaptic potentiation