ro5 br Experimental Procedures br Acknowledgments br Introdu
Introduction Cellular and molecular mechanisms underlying synaptic plasticity in the ro5 are thought to be involved in memory acquisition. Synaptic plasticity is categorised into two main types, long-term depression (LTD) and long-term potentiation (LTP) involving either a long-lasting decrease or increase in synaptic efficacy, respectively. There are two main forms of coexisting LTD at hippocampal CA1 synapses which are dependent either on synaptic NMDAR activation (NMDAR-LTD; Dudek and Bear, 1992, Mulkey and Malenka, 1992) or on mGluR activation (mGluR-LTD; Bashir et al., 1993, Bolshakov and Siegelbaum, 1994). Bath application of NMDA and the group I selective mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) can be used to induce robust NMDAR-LTD (NMDA-LTD; Lee et al., 1998) and mGluR-LTD (DHPG-LTD; Palmer et al., 1997, Fitzjohn et al., 1999), respectively. DHPG has been widely used to investigate the induction and expression mechanisms underlying mGluR-LTD in hippocampal neurons (e.g., Palmer et al., 1997, Fitzjohn et al., 1999, Fitzjohn et al., 2001, Schnabel et al., 1999, Huber et al., 2000, Rouach and Nicoll, 2003, Tan et al., 2003, Gallagher et al., 2004, Huang et al., 2004, Moult et al., 2006). A major advantage of DHPG and NMDA induced LTD is that synchronised changes at a large number of synapses facilitate the monitoring of biochemical events. Characteristically NMDAR-LTD includes the following: (1) increased postsynaptic calcium concentration [Ca2+]i (Mulkey and Malenka, 1992), (2) activation of a serine/threonine phosphatase cascade (Mulkey et al., 1993, Mulkey et al., 1994), (3) activation of GSK-3β (Peineau et al., 2007) and (4) internalisation of AMPARs via an NSF/AP2/clathrin-dependent mechanism (Nishimune et al., 1998, Lüscher et al., 1999, Luthi et al., 1999, Noel et al., 1999, Beattie et al., 2000, Man et al., 2000, Carroll et al., 2001, Lee et al., 2002, Rammes et al., 2003, Ashby et al., 2004). In contrast to NMDAR-LTD, DHPG-induced mGluR-LTD is Ca2+ independent (Fitzjohn et al., 2001) and does not involve serine/threonine phosphatases (Schnabel et al., 2001). However, mGluR-LTD is dependent on G-protein activation (Kleppisch et al., 2001, Watabe et al., 2002, Huang et al., 2004), protein tyrosine phosphatases (PTPs; Moult et al., 2002, Huang and Hsu, 2006, Moult et al., 2006), MAPK cascades (Rush et al., 2002, Gallagher et al., 2004, Huang et al., 2004, Li et al., 2007b, Moult et al., 2008), arachidonic acid (AA; Feinmark et al., 2003), phosphoinositide-3-kinase (PI3K)-Akt-mammalian target of the rapamycin (mTOR) signalling pathways (Hou and Klann, 2004), cyclinD1–cyclin-dependent kinase 4 (CDK4) complex expression (Li et al., 2007a) and postsynaptic protein synthesis (Huber et al., 2000, Huber et al., 2001, Hou and Klann, 2004). Phosphorylation is a key post-translational modification that facilitates the precise regulation of synaptic proteins necessary for modulation of synaptic transmission. Dynamic changes in phosphorylation can alter electrophysiological characteristics, protein–protein interactions and synaptic delivery or internalisation of AMPARs (reviewed in Molnár, 2008). These changes underlie the major molecular mechanisms that affect many forms of synaptic plasticity. Compared to roles of serine/threonine phosphorylation/dephosphorylation of AMPAR subunits, much less is known about the role of tyrosine phosphorylation/dephosphorylation in synaptic plasticity. While previous studies demonstrated that DHPG-LTD induction involves tyrosine dephosphorylation of AMPARs (Huang and Hsu, 2006, Moult et al., 2006), details of the underlying molecular and cellular mechanisms have not been established.
Discussion Recent studies revealed that DHPG-LTD is dependent on activation of postsynaptic PTPs which tyrosine dephosphorylate AMPARs (Huang and Hsu, 2006, Moult et al., 2006, Zhang et al., 2008). Here we established that: (1) both GluR2 and GluR3 AMPAR subunits are tyrosine phosphorylated, but only GluR2 is dephosphorylated in DHPG-LTD. (2) Tyrosine dephosphorylation of GluR2 is a specific feature of DHPG-LTD in that it does not occur in NMDA-LTD. (3) PTP-mediated dephosphorylation of cell surface AMPARs is associated with increased AMPAR endocytosis in DHPG-LTD. AMPAR tyrosine dephosphorylation may therefore be a trigger for the redistribution and internalisation of surface receptors. This study provides a better understanding of the role of PTPs during mGluR-mediated LTD induction and how they are linked to downstream postsynaptic expression mechanisms.