The role of non-canonical pathway of the membrane-tethered AKT1 kinase in regulation of LTP expression
The Serine/Threonine kinase AKT/PKB is involved in neuronal cell development, axonal growth and synaptic plasticity. Previous studies revealed a critical role of AKT in long-term plasticity and dendritic architecture. Multiple upstream activators of AKT signaling have an essential role in neuronal plasticity and memory formation. Particularly, BDNF-dependent AKT phosphorylation in hippocampus correlates with spatial memory formation, whereas in the amygdala AKT phosphorylation is associated with long-term potentiation (LTP) and fear conditioning. AKT is a connecting link in the PI3K-mTORC pathway, associated with late phase LTP (L-LTP). PI3K activity is necessary for spinogenesis, persistence of L-LTP, protein synthesis and AMPA receptor (AMPAR) insertion during the L-LTP. Activation of the PI3K/AKT/GSK3 pathway increases synaptogenesis and spinogenesis and enhances learning formation. Phospho-AKT in tandem with APPL1 is involved in spine formation. APPL1 increases the amount of active AKT in spines and synapses, and modulates AKT activity. A crucial role in regulation of L-LTP and long-term memory was demonstrated for AKT downstream effector, mTORC1, a key regulator of protein de novo synthesis. In contrast to L-LTP, evidences for the role of AKT signaling in regulation of early phase LTP (eLTP) are scarce. AKT phosphorylation time-course correlates with expression of eLTP. Scavenging of PIP3, an essential factor of AKT activation, blocks eLTP. Moreover, PI3K affects induction, expression and maintenance of eLTP. Phosphorylation of GSK-3β during LTP by the AKT inhibits long-term depression (LTD). In the NMDAR-dependent LTD, PP1 dephosphorylates and caspase-3 cleaves AKT, activating GSK3β, which sustains LTD. Hence, GSK3β is involved in downregulation of AMPARs. In contrast, a very recent finding supporting this idea shows that AKT downstream effector, PIP5K2A regulates GluR1 expression. Several other studies indicate the importance of PI3K and AKT in AMPARs upregulation.
In our Laboratory
We investigate the role of AKT kinase, its downstream pathways and upstream regulation on long-term synaptic plasticity and memory formation.
Protein analysis of alteration of protein expression levels during spatial memory formation conducted in our laboratory discovered that AKT is one of the key regulator protein involved at the phase of memory acquisition. Further, evaluating functional role of AKT, we found that AKT1 inhibition both decreases GluR2 level in hippocampal synaptosomes, eLTP expression in and AMPA/NMDA current density ratio hippocampal slices, while AKT activation acts vice versa. However, AKT1 activation without its membrane-tethering via PIP3 leads to a paradoxical strong eLTP depotentiation and AMPAR level reduction in synaptosomes, despite phosphorylation of AKT1 canonical downstream effectors, known to be involved in AMPAR expression regulation. These data indicates existence of non-canonical AKT pathway, necessary for eLTP expression. Based on these evidences, we hypothesize that membrane-tethered activated AKT (mAKT) pool, by triggering yet unknown signaling pathway regulates AMPAR membrane insertion/activity, subsequently mediating the post-synaptic mechanism of eLTP expression. If the hypothesis is correct, it will have a great impact in understanding of a wide range of phenomena depending on LTP, among others, learning and memory, and neuropathic pain.
We focus on the effects of AKT1 isoform, which is widely expressed in the hippocampus. We investigate the role of an AKT non-canonical pathway in enhancement of AMPAR current necessary for early phase LTP expression. We also study the role of interaction between the membrane and AKT regulate basal synaptic transmission and expression of early phase LTP. In parallel to functional and molecular study, we investigate which proteins are involved in non-canonical AKT signal transduction effect on LTP expression. Finally, using spatial memory behavioral paradigm and hyperalgesia model of LTP, we will attempt to show universality of AKT role in LTP expression mechanism. Potential outcome of the project may show also universality of ATK in regulation of vesicle endocytosis and membrane protein expression regulation on the long-run of the investigation.
This study uses a wide interdisciplinary approach including comprehensive electrophysiology, optical and biochemical studies, gene modification and pharmacology.
Investigation of the role of an AKT non-canonical pathway in enhancement of AMPAR current necessary for eLTP expression (Ph.D or Post-Doctoral position, A001)
Elucidation of the role of membrane-AKT interaction in the regulation of eLTP expression (Ph.D or Post-Doctoral position, A002)
Identification of the protein components of non-canonical AKT pathway and their role in early phase LTP expression regulation (Ph.D or Post-Doctoral position, A003)
Characterization of universality AKT dependent regulation of the post-synaptic mechanisms of LTP expression (Ph.D or Post-Doctoral position, A004)
Related References from our Lab
Pen Y, Borovok N, Sheinin A, Reichenstein M, Michaelevski I The role of membrane anchored AKT in regulation of basic synaptic transmission and early phase LTP expression. Hippocampus 2016; Sep;26(9):1149-67.; doi: 10.1002/hipo.22597; PMID: 27068236
Borovok N, Nesher E, Levin Y, Reichenstein M, Pinhasov A, Michaelevski I, Dynamics of hippocampal protein expression during long-term spatial memory formation. Molecular and cellular proteomics 2016 Feb; 15(2):523-41. doi: 10.1074/mcp.M115.051318.