mGluR-dependent LTD

1. families- 3 Groups

   Annotations:

   • each group have a different downstream signalling mechanisms
   1. Group I +PLC

      Annotations:

      • GluR1,5 Gq coupled
   2. Group II -cAMP

      Annotations:

      • GluR2,3 Gi/o
   3. Group III

      Annotations:

      • GluR4,6,7,8

Annotations:

• Because of conflicting results from studies, it cannot be conclusively stated whether hippocampal mGluR-LTD expression is presynaptic or postsynaptic. Inconsistent results from these studies could be due to differences in experimental design or temperature or to differences in the age of the animal used. It is entirely possible that both presynaptic (neurotransmitter release) and postsynaptic (AMPAR internalization) expression mechanisms occur. It is generally thought that induction involves the postsynaptic activation of PTPs and other signaling cascades, which could include a retrograde signaling molecule that passes back to the presynaptic neuron. If both pre- and postsynaptic mechanisms do contribute to mGluR-LTD, it may be that they can occur simultaneously or that differing experimental conditions will favor one mechanism over the other. 

1. presynaptic

   Annotations:

   • observed during synaptically induced LTD and DHPG-LTD
   1. photolysis of caged glutamate

      Annotations:

      • Negative postsynaptic changes include a lack of alteration in sensitivity to uncaged L-glutamate upon DHPG-LTD induction (Rammes et al., 2003). However, these experiments were performed at relatively low temperatures (22–24°C), which may prevent the detection of postsynaptic changes such as receptor internalization
      1. AMPAR diffused?

         Annotations:

         • The lack of change in the response to uncaged glutamate, which act on both spines and dendrites, can be reconciled with a postsynaptic model if, instead of being internalized, AMPA receptors diffuse laterally to extrasynaptic sites (Moult et al., 2006).
      2. low temp-24-25
   2. NMDAR and AMPAR responses

      Annotations:

      • NMDA and AMPA receptor components of synaptic responses are equally depressed (Watabe et al., 2002),
   3. increased PPF
   4. decrease in frequency, not amplitude of mEPSCs
   5. AMPAR removed- silent synapses?

      Annotations:

      • An alternative explanation is that AMPA receptors are removed to create silent synapses. If this occurs preferentially at high-Pr synapses owing to the requirement for activity to drive the movement, the changes in paired-pulse facilitation would simply reflect this postsynaptic alteration
   6. increase in the EPSC amplitude coefficient of variation (CV)
   7. increase in neurotransmitter release

      Annotations:

      • indicated by zinc fluorescence detection of exocytosis (Qian and Noebels, 2006). 
      1. Xu et al (2013)

         Annotations:

         • rat HPC using biotinylation of vesicular transporters in vesicles fused with presynaptic membranes during neurotransmitter release-  to selectively quantify the spontaneous or evoked release of glutamate or GABA at their respective synapses >>mGluR-LTD associated with- increased presynaptic release of glutamate, despite reduced miniature EPSCs measured with whole-cell recording
         1. blockade of AMPA receptor (AMPAR) endocytosis

            Annotations:

            • eliminated LTD-associated increase in presynaptic release >>Thus, our work not only demonstrates that mGluR1/5-mediated LTD is associated with increased endocytosis of postsynaptic AMPARs but also reveals an unexpected homeostatic/compensatory increase in presynaptic release.
   8. would require retrograde signaling molecule
      1. metabolite of arachidonic acid
         1. unlikely because this pathway is Ca mediated

            Annotations:

            • This is inconsistent with previous evidence indicating that mGluR-LTD is Ca2+-independent, but implies that there may be alternative retrograde signaling molecules that can act in the absence of Ca2+. 
      2. presynaptic targets

         Annotations:

         • Potential presynaptic targets of a retrograde signal are voltage-gated ion channels or constituents of the presynaptic release machinery 
         • For example, the presynaptic S-type K+ channel is activated by 12(S)-HPETE, which leads to inhibition of neurotransmitter release by decreasing Ca2+ influx(Feinmark et al., 2003).
2. postsynaptic
   1. receptors trafficking

      Annotations:

      • evidence that DHPG-induced LTD involves removal of both AMPA and NMDA receptors from synapses (Snyder et al., 2001).
   2. PTP inhibitors- postsynaptic

      Annotations:

      • mGluR-dependent LTD, induced by DHPG, is blocked by postsynaptically applied inhibitors of protein tyrosine phosphatases (PTPs) and of the actin cytoskeleton (Moult et al,2006)
      1. retrograde signalling mediated by PRP

         Annotations:

         • These postsynaptic manipulations using PTP inhibitors blocked the changes in paired-pulse facilitation and miniature frequency, suggesting a retrograde signaling process that is dependent on PTPs and the actin cytoskeleton.
   3. reduction in the amplitude of mini-EPSCs
3. developmental switch

   Annotations:

   • It is proposed that mGluR-LTD is presynaptically expressed in neonatal synapses, because there is no change in surface expression of AMPARs, and it seems to be independent of protein synthesis. During synapse maturation, there is a developmental switch involving mGluR regulation of AMPAR trafficking via the production of new synaptic proteins. At immature synapses, regulation of presynaptic release probability rather than AMPAR internalization may be a more efficient way of depressing transmission at synapses (Nosyreva and Huber, 2005). 

1. stimulation induced LTD
   1. PP-LFS

      Annotations:

      •  adult brain slices using paired-pulse LFS (PP-LFS) comprising 900 pairs of stimuli delivered at 1 Hz with 50-ms intervals
      1. nonspecific mGluR anta

         Annotations:

         • using a paired pulse LFS protocol- they demonstrated that the application of a GluR antagonist blocked the LTD compared to controls
         1. block LTD
      2. may be more NMDAR-dependent

         Annotations:

         • see protocols- PP vs single pulse
      3. Age of animals
         1. postnatal (21-28 days)
            1. mGluR activation alone

               Annotations:

               • only mGluR activation was required for PP-LFS-induced LTD (Huber et al., 2000), 
         2. adult
            1. mGluR anta alone

               Annotations:

               • PP-LFS-induced LTD can also be blocked by mGluR antagonists alone (Moult et al., 2008). 
      4. Group 1

         Annotations:

         • The conclusions drawn from studies investigating the role of group I mGluRs in CA1 synaptic plasticity can differ because of the various specificities of subunit antagonists and differences in the durations of in vitro and in vivo experiments.
         1. GluR1 anta

            Annotations:

            • Moult et al (2008)
            1. doesn't block LTD
         2. GluR5anta

            Annotations:

            • Moult et al (2008)
            1. blocks LTD
            2. late phase LTD?

               Annotations:

               • mGlu5 receptors convert short-term depression into LTD, indicating that this subunit is necessary to mediate the late-phase of this form of synaptic plasticity
               1. dependent on protein synthesis
      5. Initial studies
         1. nonspecific mGluR /group 1 mGluR anta (CPCCOEt) + AMPAR/ kainate anta

            Annotations:

            • Although the mGluR antagonist MCPG and the group I mGluR antagonist can block synaptically induced LTD, initial studies showed this only occurs if the AMPA/kainate receptor antagonist (CNQX) is also applied
            1. block LTD

               Annotations:

               • Kemp and Bashir, 1999
         2. AMPAR/ kainate anta/ nonspecific mGluR (MCPG) alone
            1. don't block LTD

               Annotations:

               • Kemp and Bashir, 1999
            2. Kemp and Bashir, 1999
      6. Protein synthesis dependent?
         1. blocked by protein synthesis inhibitor

            Annotations:

            • Synaptically driven mGluR-dependent LTD induced by low-frequency trains of paired pulses was also blocked by anisomycin (Hüber et al., 2000).
         2. Protein synthesis independent

            Annotations:

            • Moult et al., 2008
         3. Kinases
            1. activation of kinases MAPK and ERK

               Annotations:

               • P-LFS may also involve activation of the mitogen activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK), which facilitates regulation of protein synthesis (Gallagher et al., 2004). 
            2. MAPK and PTP inhibitors

               Annotations:

               • Furthermore, the effects of PP-LFS is blocked by inhibitors of both p38 MAPK and protein tyrosine phosphatases (PTPs), suggesting that these signaling cascades may also be fundamental (Moult et al., 2008).
   2. LFS
      1. LTD blocked by calcium chelators
         1. there is a retrograde messenger?
            1. possibly AA
      2. in-vitro, but not in-vivo
   3. Group I vs II anta

      Annotations:

      • although group I mGluR antagonists are generally more effective at blocking mGluR-dependent LTD in area CA1 in vitro, group II antagonists depress the late component of LTD in vivo (Manahan- Vaughan, 1997).
2. Agonist induced LTD

   Annotations:

   • DHPG-LTD depends predominantly on mGlu5 receptor activation, although the mGlu1 receptor may partly contribute to LTD induction.
   1. group I mGluR agonist (DHPG)

      Annotations:

      • Brief exposure to the group I mGluR agonist dihydroxyphenylglycine (DHPG) induces a reassuringly robust LTD in adult slice preparations in both area CA1 (Palmer et al., 1997) (Fig 10.17E) and the dentate gyrus (Camodeca et al., 1999).
      1. CA1
      2. DG
      3. Receptor anta

         Annotations:

         • This form of LTD, probably mediated by mGluR5 receptors, is blocked by group I or broad-spectrum mGluR antagonists but not by NMDA receptor antagonists
         1. blocked -broad spectrum mGluR antagonists LY341495 and MCPG
         2. blocked- specific mGluR group 1/ mGlu5 anta
         3. mGlu1 anta- block short-term not long-term phase

            Annotations:

            • generally has no effect on the long-term phase, although the short-term phase is reduced (Fitzjohn et al., 1999; Huang et al., 2004). 
         4. not blocked- NMDAR anta
         5. requires sustained activation of mGlu5

            Annotations:

            • mGluR-LTD involves sustained activation of mGlu5 receptor, supported by the fact that broad spectrum mGluR antagonists reverse DHPG-LTD several hours after induction (Palmer et al., 1997; Fitzjohn et al., 1999; Watabe et al., 2002;Huang and Hsu, 2006). 
      4. G protein inhibitor- blocked LTD induction

         Annotations:

         • Intracellular injection of a G protein inhibitor into CA1 pyramidal cells suppressed the ability of DHPG to induce LTD (Watabe et al., 2002).
      5. Protein synthesis inhibitor- reduced LTD

         Annotations:

         • In the presence of the protein synthesis inhibitor anisomycin, the depression induced by DHPG quickly attenuates, and responses recover to baseline within 60 minutes (Hüber et al., 2000).
      6. Ca- doesn't block
         1. Intracellular injection of BAPTA, (Ca chelator)

            Annotations:

            • Intracellular injection of BAPTA, on the other hand, doesn't block LTD (Fitzjohn et al., 2001b),
      7. mGlu5-KO

         Annotations:

         • The absence of DHPG-LTD in mGlu5receptor knockout (KO) mice further highlights the prominent role of mGlu5receptor in LTD induction in the CA1 region of the hippocampus (Huber et al., 2001).
         1. no LTD
      8. mGlu1/5 alone

         Annotations:

         • However, one study has suggested that activation of either mGlu1 or mGlu5 receptor is not sufficient to induce DHPG-LTD, even though LTD is abolished in mGlu5 receptor KO mice (Volk et al., 2006).
         1. not sufficient
      9. Ideal- removal of comfounding GABA and NMDAR

         Annotations:

         • For in vitro adult hippocampal slice preparation, DHPG-LTD is usually induced using an extracellular medium that is Mg2+-free or contains the GABAA receptor antagonist picrotoxin (Palmer et al., 1997). Under these conditions, mGluR-LTD is not prevented by application of the NMDAR antagonist D-2-amino-5-phosphonopentanoate (AP-5). The addition of picrotoxin removes any influence from inhibitory neurotransmission at GABAergic synapses. Thus, the overall excitability of the slice is increased, which lowers the threshold for induction of DHPG-LTD (Palmer et al., 1997).
         1. reduced threshold for induction
      10. doesn't require presynaptic stimulation

          Annotations:

          • DHPG-LTD induction does not require presynaptic stimulation (Fitzjohn et al., 1999).
   2. Broad spectrum mGluR agonists (ACPD and quisqualate )

      Annotations:

      • (Schoepp et al., 1999
   3. selective mGluR5 agonist (CHPG)

      Annotations:

      • Palmer et al., 1997; Fitzjohn et al., 1999; Huber et al., 2000, 2001
   4. Lodge et al (2013)
      1. Group II
         1. agonist induced LTD is reversed by anta

            Annotations:

            • activation of group II mGlu receptors in the temporo-ammonic pathway (TAP) and mossy fibre pathway within the hippocampus and in the cortical input to neurons of the lateral amygdala induces an LTD that is reversed by LY341495, a group II mGlu receptor antagonist
         2. temporo-ammonic pathway (TAP) and mossy fibre pathway, lateral amyg
      2. Group III- mGlu8
         1. agonist induced LTD is revered by anta

            Annotations:

            • activation of group III mGlu8 receptors induces an LTD at lateral perforant path inputs to the dentate gyrus and that this LTD is reversed by MDCPG, an mGlu8 receptor antagonist
         2. lateral perforant path- DG input
      3. LTD- induction region specific?
3. DHPG vs PP-LFS
   1. similar mechanisms and signalling

      Annotations:

      • It is likely that DHPG-LTD and PP-LFS-induced LTD involve similar expression mechanisms, because the two forms of LTD occlude each other (Huber et al., 2001). Furthermore, it is evident that induction of both DHPG-LTD and PP-LFS-induced LTD employ similar signaling cascades involving MAPKs and PTPs (Rush et al., 2002; Huang et al., 2004b; Moult et al., 2008).

1. signalling

   Attachments:

   • mGluR LTD
   1. Protein phosphastases

      Attachments:

      • LTD
      1. PTP
         1. retrograde signalling- PTP?
         2. Transient of persistent PTP?

            Annotations:

            • controversy regarding whether transient (Moult et al., 2002,2006) or persistent (Huang and Hsu, 2006) PTP activation is required. This may be due to differences in experimental design
         3. PTP inhibitors
            1. Patch clamp- whole-cell recordings- PAO/OV

               Annotations:

               • injecting the PAO/ OV intracellularly into the cell via the pipette >>blocked LTD
            2. Moult et al 2006
               1. OV- blocked a decrease in the level of tyrosine phosphorylation of GluR2 AMPA

                  Annotations:

                  • Moult et al (2006) can induce LTD- DHPG treatment of hippocampal slices was associated with a decrease in the level of tyrosine phosphorylation of GluR2 AMPA receptor (AMPAR) subunits, an effect blocked by orthovanadate
               2. OV- AMPAR receptor no.

                  Annotations:

                  • OV- blocked the ability of DHPG to reduce the number of AMPA receptor clusters on the surface of dendrites
            3. Moult et al (2008)
               1. Extracellular- non-selective phosphatase inhibitor (OV)

                  Annotations:

                  • Moult et al (2008) fEPSP measurements- extracellular recordings? blocks LTD after a PP-LFS stimulation protocol- 
               2. selective inhibitor (PAO)

                  Annotations:

                  • blocks LTD more than the nonselective inhibitor Moult et al (2008)
                  1. PPF and CV

                     Annotations:

                     • also block the changes in paired-pulse facilitation and coefficient of variation
         4. mechanism
            1. dephosphorylated AMPAR GluA2, not GluA1/3

               Annotations:

               • AMPAR GluA2 subunit, but not GluA1 or GluA3, is tyrosine dephosphorylated during mGluR-LTD (Moult et al., 2006; Gladding et al., 2009). 
               1. PTP- specific to mGluR-LTD

                  Annotations:

                  • AMPAR tyrosine dephosphorylation is specific to mGluR-LTD, because only GluA1 serine dephosphorylation was observed during NMDA-LTD (Gladding et al., 2009). 
               2. AMPAR interactions with scaffolding and effector proteins- stability

                  Annotations:

                  • Regulation of tyrosine phosphorylation is likely to modulate AMPAR interactions with scaffolding and effector proteins, which subsequently alters receptor stability at the synapse
               3. GluA2

                  Annotations:

                  • tyrosine dephosphorylation of the GluA2 subunit triggers the endocytosis of surface AMPARs (Gladding et al., 2009).
                  1. DHPG- LTD- increase AMPAR endocytosis, reduced phospho

                     Annotations:

                     • the rate of AMPAR endocytosis is increased, and the tyrosine phosphorylation of surface but not intracellular AMPARs is reduced. Hence, it is postulated that tyrosine dephosphorylation of surface AMPARs initiates their redistribution away from the synapse via lateral diffusion and internalization. 
                  2. striatal-enriched tyrosine phosphatase (STEP)

                     Annotations:

                     • It has recently been discovered that the PTP that dephosphorylates the GluA2 subunit during mGluR-LTD is the striatal-enriched tyrosine phosphatase (STEP) (Zhang et al., 2008). 
                     1. MAPK, PI3K pathways

                        Annotations:

                        • mGluR activation leads to elevated STEP expression via activation of both MAPK and phosphoinositide-3-kinase (PI3K) pathways. 
                     2. increase activation expression- AMPAR endocytosis
                     3. ERK1/2 negative feedback

                        Annotations:

                        • Although ERK1/2 stimulates STEP translation, STEP can also dephosphorylate and inactivate ERK1/2 in addition to p38 MAPK (Muñoz et al., 2003; Paul et al., 2003). Because ERK1/2 and p38 MAPK signaling cascades are involved in mGluR-LTD induction (Rush et al., 2002; Gallagher et al., 2004; Huang et al., 2004b; Moult et al., 2008), STEP activation may be regulated by a feedback mechanism (Zhang et al., 2008).
                     4. transgenic alzheimers mice- increase expression

                        Annotations:

                        • STEP activity is increased in transgenic mouse models of Alzheimer's disease (Chin et al., 2005; Snyder et al., 2005), 
         5. PTK inhibitors

            Annotations:

            • Although PTK inhibitors have no significant effect on DHPG-LTD, they do prevent the block mediated by PTP inhibitors (Moult et al., 2006).
            1. NMDAR-LTD and mGluR-LTD

               Annotations:

               • A similar effect of PTK inhibitors have been observed with regard to NMDAR-LTD (Coussens et al., 2000). 
            2. invovled in parallel pathways

               Annotations:

               • It is possible that mGluR-LTD requires the activation of two parallel independent pathways (Moult et al., 2006, 2008).   Whereas PTP activation is required for LTD induction, a parallel pathway may mediate LTD expression by modulating AMPAR trafficking. Parallel activation of p38 MAPK may induce AMPAR internalization by stimulating the formation of the GDI-Rab5 complex involved in clathrin-dependent endocytosis (Rush et al., 2002; Huang et al., 2004b; Moult et al., 2008). 
               1. PTP-mediated dephosphorylation of AMPARs
               2. p38 MAPK cascade
               3. PTP inhibition

                  Annotations:

                  • PTP inhibition favors PTK phosphorylation of AMPARs and prevents mGluR-LTD induction (Moult et al., 2006, 2008). 
               4. PTK inhibiton

                  Annotations:

                  • PTK inhibition does not facilitate LTD induction because it is also necessary to activate the parallel pathway. PTK inhibitors may reverse the effect of PTP inhibitors, because PTK activation is required for the PTP inhibitor block.
      2. PP1/ PP2A

         Annotations:

         • inhibitors of the ser/thr phosphatases that are involved in NMDAR-dependent LTD (PP1/PP2A and PP2B) do not inhibit DHPG-induced LTD; rather, PP1/PP2A inhibitors cause a small facilitation of the effect (Schnabel et al., 2001).
         1. NOT in mGluR- LTD
   2. PI3K-Akt mTOR

      Annotations:

      • mGluRs may be coupled to the PI3K cascade via interactions with Homer and the cytoplasmic GTPase PI3K enhancer-L (Rong et al., 2003). Formation of the mGluR-Homer-PI3K enhancer complex is necessary to stimulate translation of 5′TOP containing mRNAs via activation of the PI3K pathway (Ronesi and Huber, 2008)
      1. cap-dependent translation

         Annotations:

         • It is believed that both the PI3K-Akt-mTOR and the ERK-MEK pathways mediate cap-dependent translation during mGluR-LTD (Banko et al., 2006).
      2. eukaryotic initiation factor 4F (eIF4F)

         Annotations:

         • mGluR-LTD triggers an increase in the formation and binding of the eukaryotic initiation factor 4F (eIF4F) complex to the mRNA 5′TOP sequence.
         1. eIF4E-binding protein (4E-BP)

            Annotations:

            • This is dependent on binding of eIF4E and eIF4G to the complex, which is regulated by phosphorylation of eIF4E-binding protein (4E-BP). mGluR activation facilitates the binding of eIF4E to eIF4G by phosphorylating 4E-BP2 and preventing eIF4E inhibition
         2. Mnk1 -ERK-MEK signaling cascade

            Annotations:

            • Furthermore, initiation of mRNA translation is dependent on phosphorylation of eIF4E by Mnk1 (Gingras et al., 1999), which is also regulated by the ERK-MEK signaling cascade (Banko et al., 2006).
      3. translation

         Annotations:

         • Translational events can additionally be modulated by mTOR-mediated phosphorylation of ribosomal protein S6 via activation and phosphorylation of S6 kinase (S6K) (Dufner and Thomas 1999; Gingras et al., 2001) (Fig. 2). 
         • The  PI3K/AKT/mTOR/p70S6K1 pathway ediate the regulation of the expression and translocation of the cyclinD1-CDK4 complex in the hippocampus. It is postulated that mGluR-LTD activates the PI3K/AKT/mTOR cascade, which stimulates the cyclinD1-CDK4 complex via the retinoblastoma (Rb)/E2F1 pathway leading to modulation of protein synthesis (Li et al., 2007a) (Fig. 2).
      4. parallel to ERK-MRK pathway
   3. Regulation of Gene Expression
      1. MAPK and PI3K signaling pathways
         1. NF-κB

            Annotations:

            • NF-κB is a further transcription factor that is activated in mGluR-LTD via PI3K, Ras, and p38 MAPK signaling pathways (O'Riordan et al., 2006). 
            1. long-term LTD

               Annotations:

               • The NF-κB transcription factor C-Rel is specifically required for long-term maintenance of hippocampal mGluR-LTD. Although previous studies have indicated that mGluR-LTD involves translational rather than transcriptional regulation, this may be because synaptic changes were monitored at an early phase (<90 min) rather than at a late phase (2–3 h) of LTD (Huber et al., 2000; O'Riordan et al., 2006). 
            2. increases gene expression

               Annotations:

               • In contrast to other transcription factors, NF-κB was the first to be synaptically localized (Korner et al., 1989) and upon synaptic activation is rapidly trafficked from the synapse to the nucleus (Meberg et al., 1996; Wellmann et al., 2001;Meffert et al., 2003). In mGluR-LTD stabilization, NF-κB may act as a signal messenger to facilitate an increase in specific gene expression in response to synaptic activity (O'Riordan et al., 2006).
         2. -26-specific (ETS) domain transcription factor Elk-1,

            Annotations:

            • DHPG-LTD involves ERK-mediated activation of the E-26-specific (ETS) domain transcription factor Elk-1, which is a tertiary complex factor (Wang et al., 2004; Mao et al., 2005). 
         3. CREB

            Annotations:

            • ERK can also activate cAMP response element-binding (CREB), which, with Elk-1, facilitates expression of the gene c-Fos (Mao et al., 2005). 
   4. Mitogen-Activated Protein Kinase (MAPK)

      Annotations:

      • It has been demonstrated that mGluR-LTD involves all three MAPK subclasses: p38 MAPK, JNK,ERK
      • The evidence indicates that mGluR activation can couple to different signaling mechanisms dependent on the experimental conditions
      • The MAPK signaling cascades are important for the induction and expression of mGluR-mediated LTD in the hippocampus. The p38 MAPK cascade links mGluR activation to AMPAR internalization via its coupling to endocytotic machineries. ERK can modulate neuronal protein synthesis but may also facilitate a decrease in AMPAR-mediated synaptic transmission by triggering calpain-mediated GluA1 proteolysis. JNK is also involved in the expression of mGluR-LTD by activating specific transcription factors via changes in post-translational modifications.
      1. p38 MAPK
         1. CA1- Rap1–MAPK kinase 3/6–p38 MAPK cascade

            Annotations:

            • It has been implied that mGluR-LTD induction involves the repressor activator protein 1 (Rap1)–MAPK kinase 3/6–p38 MAPK cascade in the CA1 region (Huang et al., 2004b). 
            1. AMPAR internalization

               Annotations:

               • The pathway is coupled to endocytotic machineries and is thought to stimulate AMPAR internalization downstream of mGlu5 receptor activation (Fig. 1). 
            2. ras and rap- GTPases

               Annotations:

               • Ras and Rap are important GTPases that control AMPAR trafficking at synapses and are regulated by activators, guanine-nucleotide exchange factors and inactivators, and GTPase-activating proteins (Zhu et al., 2002). 
               1. rap1
                  1. MAPK

                     Annotations:

                     • upon mGlu5 receptor activation, Rap1 and sequentially MAPK kinase 3/6 are activated by the release of Gβγ subunits. This leads to p38 MAPK activation, which promotes AMPAR internalization via the formation of the GDI-Rab5 complex (Cavalli et al., 2001) 
         2. DG- MAPK-PKC- tumor necrosis factor-receptor 1

            Annotations:

            • mGluR-LTD induction in the dentate gyrus is also thought to depend on p38 MAPK and PKC activation, which may also involve the tumor necrosis factor-receptor 1 (Rush et al., 2002; Wang et al., 2007). 
      2. Jun N-terminal kinase (JNK)

         Annotations:

         • required under some conditions
         1. JNK KO mice

            Annotations:

            • both PP-LFS and DHPG-LTD were impaired in the CA1 region of JNK KO mice (Li et al., 2007b) 
         2. increase in phosphorylation; JNK1 , ATF2, c-Jun

            Annotations:

            • increase in the phosphorylation levels of the JNK1 substrates activating transcription factor 2 (ATF2) and c-Jun
            1. c-jun expression

               Annotations:

               • Expression of the c-Jun gene is mediated by ATF2 and c-Jun transcription factors that combine together to form heterodimers (van Dam et al., 1995). However, in this study there was no change in c-Jun expression levels, indicating that post-translational modifications rather than mRNA transcription were necessary for mediating DHPG-LTD (Li et al., 2007b). A further study supports a role of c-Jun in mGluR-LTD expression in neurons (Yang et al., 2006). Activator protein-1 mediated gene expression is facilitated by a process that involves dimerization of c-Jun and Fos (Schwarzschild et al., 1997; Yang et al., 2006).
      3. ERK
         1. induction and regulation of protein synthesis

            Annotations:

            • , but not p38 MAPK, is required for the induction and regulation of synaptic protein synthesis (Gallagher et al., 2004) 
         2. rap1-MAPK/MEK
            1. Downstream
               1. ribosomal S6 kinase-1 (RSK1)

                  Annotations:

                  • downstream effector ribosomal S6 kinase-1 (RSK1), which is known to be a key regulator of neuronal protein synthesis in response to synaptic activity (Angenstein et al., 1998). 
               2. m-calpain

                  Annotations:

                  • The ERK/MAPK signaling cascade is also linked to regulation of m-calpain, an isoform of the large calpain subunit (Perrin and Huttenlocher, 2002). It is thought that in addition to phosphorylation, calpain-mediated truncation of iGluR C-terminal tails is important in mediating synaptic plasticity (Guttmann et al., 2001). 
                  1. reduction of AMPAR transmission

                     Annotations:

                     • Although it has not been directly shown, mGluR-LTD may involve a decrease in AMPAR-mediated synaptic transmission as a result of an increase in AMPAR calpain proteolysis (Fig. 1).
                  2. NMDA

                     Annotations:

                     • High NMDA concentrations lead to an increase in GluA1 proteolytic cleavage, which is differentially regulated by GluA1 phosphorylation by CaMKII and protein phosphatases 1 and 2A (Yuen et al., 2007). It has also been demonstrated that GluA1 phosphorylation by Fyn decreases calpain-mediated proteolysis (Rong et al., 2001). 
   5. Proteases
   6. Endocannabinoids

      Attachments:

      • Endocannabinoids in synaptic plasticity
      1. retrograde signal
      2. presynaptic CB1R

         Annotations:

         • its activation leads to a reduction in the probability of neurotransmitter release
      3. excitatory synapses

         Annotations:

         • DHPG-LTD of excitatory transmission is not believed to involve eCB production, although the short-term depression seen during DHPG application is reduced by CB1 receptor antagonism (Rouach and Nicoll, 2003). 
      4. inhibitory synapses

         Annotations:

         • LTD at inhibitory synapses on CA1 pyramidal neurons is dependent on mGluR-stimulated eCB production (for review, see Chevaleyre et al., 2006).
   7. Protein synthesis
      1. mGluR-LTD may be protein synthesis independent
      2. mechanisms
         1. regulated by mGluR-homer itneraction
         2. synthesized close to the synapse

            Annotations:

            • To facilitate rapid modulation of synaptic transmission, key mediator proteins can be synthesized in close proximity to the synapse (Sutton and Schuman, 2006). mRNA-protein complexes can therefore be transported along the dendrite via interactions with microtubule filaments in the cytoskeleton. 
            1. evidence

               Annotations:

               • group I mGluR activation mediates increased translation of AMPAR subunit mRNA in nearby regions of the dendrite (Grooms et al., 2006). In response to mGluR-LTD induction, local translation facilitates rapid protein recruitment to the synapse such that synaptic transmission can be efficiently modulated
               1. KO's
                  1. 4E-BP2 KO mice- increase mGluR-LTD

                     Annotations:

                     • indicating that 4E-BP2 has an important role in negatively regulating synaptic activity (Banko et al., 2006). 4E-BP2 is phosphorylated by the PI3K-Akt-mTOR pathway, which facilitates formation of the eIF4F translation initiation complex (Gingras et al., 1999). Modulation of 4E-BP2 phosphorylation is therefore important for regulation of translation during mGluR-LTD (Banko et al., 2006).
               2. inhibitors
         3. STEP translation
            1. mGluR activation- using agonists?

               Annotations:

               • mGluR activation facilitates an increase in STEP translation in synaptosomes via activation of MAPK and PI3K pathways
            2. stimulating MAPK/ PI3K pathways

               Annotations:

               • dendritic protein synthesis is regulated by stimulation of both signaling pathways (Wang and Tiedge, 2004). 
      3. Main roles:
         1. AMPAR endocytosis
         2. producing a retrograde signaling molecule

            Annotations:

            • that is able to regulate presynaptic neurotransmitter release
      4. Evidence
         1. isolated in vitro preparation
         2. inhibitors
      5. fragile X mental retardation protein (FMRP)

         Annotations:

         • expressed in neuronal dendrites

         Attachments:

         • mGluR LTD
         1. mRNA translation

            Annotations:

            • FMRP forms a protein complex with polyribosomes at specific mRNA sites and acts as a negative regulator of mRNA translation
            1. adaptor protein PSD-95

               Annotations:

               • mGluR stimulation leads to an increase in PSD-95 translation, which is dependent on regulation by FMRP (Todd et al., 2003).
               1. mGluR stimulation

                  Annotations:

                  • mGluR stimulation leads to an increase in PSD-95 translation, which is dependent on regulation by FMRP (Todd et al., 2003).
               2. stabilized by FMRP

                  Annotations:

                  • FMRP mediates stabilization of PSD-95 mRNA through direct interactions with its 3′ untranslated region (Zalfa et al., 2007). 
                  • This stabilization is enhanced by mGluR activation, implying that FMRP mediates the regulation of key synaptic proteins under both basal and stimulated conditions. 
            2. AMPAR subunits GluA1/GluA2 and CaMKIIα
            3. negative regulation

               Annotations:

               • Furthermore, modulation of AMPAR internalization is dependent on negative regulation of protein synthesis by FMRP (Nakamoto et al., 2007). FMRP is therefore crucial for preventing mGluR stimulation leading to excessive AMPAR endocytosis via overactivation of signaling cascades.
               1. Arc/Arg3.1

                  Annotations:

                  • Arc/Arg3.1 synthesis is linked to increased AMPAR endocytosis and down-regulation of AMPAR-mediated synaptic activity upon LTD induction
                  • Under basal conditions, Arc/Arg3.1 translation is negatively regulated by FMRP. Upon mGluR-LTD induction, FMRP is dephosphorylated by PP2A, which reduces its binding to target mRNAs (Narayanan et al., 2007). This facilitates rapid de novo local synthesis of specific mRNAs at close proximity to the synapse. 
                  1. Park et al (2008)

                     Annotations:

                     •  propose a model in which eEF2K-eEF2 and FMRP coordinately control the dynamic translation of Arc/Arg3.1 mRNA in dendrites that is critical for synapse-specific LTD
                     1. Double Arc/Arg3.1/Fmr1 KO

                        Annotations:

                        • Fmr1 KO Disrupts Rapid, but Not Delayed, Induction of Arc/Arg3.1 Protein Double Arc/Arg3.1/Fmr1-  KOArc/Arg3.1 Is Required for mGluR-LTD and PP-LFS LTD in Fmr1 KO Mice (albation of it blocks LTD)
                     2. mGluR-LTD and PP-LFS LTD Require Arc/Arg3.1
                     3. AMPA Receptor Endocytosis Requires Arc/Arg3.1
                     4. mGluR Induces Rapid Translation of Preexisting Arc/Arg3.1 mRNA
                     5. eEF2K KO Neurons

                        Annotations:

                        • mGluR-LTD and PP-LFS LTD Rapid De Novo Arc/Arg3.1 Translation Is Selectively Absent in eEF2K KO Neurons
            4. EF1A and S6

               Annotations:

               • mGluR-LTD leads to down-regulation of FMRP, which permits the synthesis of key proteins such as EF1A and S6 necessary for LTD induction
            5. eEF2K-eEF2
         2. Evidence
            1. FMRP expression enhanced upon mGluR activation
            2. Fmr1-KO mice

               Annotations:

               • hippocampal slices from Fmr1-KO mice showed increased mGluR-LTD (Huber et al., 2002), indicating that FMRP is important for modulation of protein synthesis upon synaptic activation
               1. LTD is protein synthesis-independent

                  Annotations:

                  • In contrast to wild-type mice, the LTD in Fmr1-KO mice is independent of protein synthesis and activation of the ERK signaling cascade (
            3. wild-type mice

               Annotations:

               • mGluR-LTD involves a rapid increase in FMRP translation that is subsequently ubiquitinated and degraded (Hou et al., 2006). Degradation of FMRP permits the translation of FMRP-targeted mRNAs and thus provides a dynamic mechanism for regulating protein synthesis during mGluR-LTD.
               1. FMRP is degraded
         3. fragile X syndrome (FXS)
            1. cognitive deficits

               Annotations:

               • cognitive deficits that may be due to impairments in synaptic glutamate signaling dependent on key effector and adaptor proteins such as PSD-95 (Bear et al., 2004; Koukoui and Chaudhuri, 2007). 
            2. Anxiety and epilepsy symptoms

               Annotations:

               • Anxiety and epilepsy symptoms are also elevated in patients with FXS andFmr1-KO mice, which could be due to disrupted interactions between mGluRs and the short isoform Homer1a (Penagarikano et al., 2007). Homer1a competes with the longer Homer isoforms for mGluR binding (Xiao et al., 1998; Fagni et al., 2002), which is important for modulation of mGluR-mediated synaptic transmission in the hippocampus (Kammermeier and Worley, 2007). Because Homer1a can protect against induction of epilepsy, anxiety, and pain (Szumlinski et al., 2006), it is postulated that mGluR interactions with both short and long Homer isoforms are impaired in FXS (Penagarikano et al., 2007).
2. hippocampal Ca-independence + kinases

   Annotations:

   • It can therefore be concluded that group I mGluR-LTD induction in the hippocampus is Ca2+-independent and does not involve the typical intracellular signaling cascade that is normally associated with group I mGluR activation. The molecular mechanisms are thought to differ substantially between different brain regions as demonstrated by the fact that LTD induction is Ca2+-dependent in both the perirhinal cortex and the cerebellum. Although DHPG-LTD and synaptically induced LTD in the CA1 share common molecular mechanisms, it should be kept in mind that some differences do exist.
   1. Ca
      1. Intracellular Ca
         1. intracellular Ca2+ depletion- no affect

            Annotations:

            • Furthermore, it was demonstrated that Ca2+ release from intracellular stores was not necessary for DHPG-LTD induction because intracellular Ca2+ depletion had no effect (Schnabel et al., 1999a; Fitzjohn et al., 2001). 
         2. Reduction in intra Ca following DHPG-LTD?

            Annotations:

            • DHPG-LTD may induce a decrease in the intracellular Ca2+ levels of the presynaptic cell (Watabe et al., 2002).
      2. Extracellular Ca
         1. Ca chelators e.g. BAPTA- don't block LTD

            Annotations:

            • DHPG-LTD induction is not prevented by the intracellular Ca2+ chelator BAPTA and does not depend on extracellular Ca2+ (Fitzjohn et al., 2001).
      3. other brain regions
         1. PRH

            Annotations:

            • in the perirhinal cortex, mGluR-LTD induction is dependent on interactions between the neuronal Ca2+sensor protein (NCS-1) and protein interacting with C kinase (PICK1) (Jo et al., 2008). The NCS-1-PICK1 complex associates with PKC near the plasma membrane, which could facilitate AMPAR endocytosis via phosphorylation of the GluA2 AMPAR subunit
         2. Cerebellum

            Annotations:

            • cerebellar parallel fiber-stellate cell synapses, mGluR and GABABR activation stimulates a decrease in Ca2+-permeable AMPARs (Kelly et al., 2009). 
   2. Kinases
      1. CamKII

         Annotations:

         • It is known that Ca2+/calmodulin-dependent protein kinases II (CaMKII) has an important role in LTP induction, hence it may be down-regulated during LTD. 
         1. CaMKII anta- enhanced DHPG-LTD at CA3:CA1 synapses

            Annotations:

            • DHPG-LTD at CA3:CA1 synapses is enhanced in the presence of the CaMKII antagonist KN-62 (Schnabel et al., 1999b). 
         2. stabilizes mGlu5 receptor expression
            1. PKC-phosphorylation of C-terminal at ser901 inhibits CaM binding>> reduced receptor expression

               Annotations:

               • This interaction is modulated by PKC-mediated phosphorylation of the intracellular C terminus of mGlu5 at serine 901 after receptor stimulation. Ser901 phosphorylation inhibits mGlu5 binding to CaM, which leads to reduced mGlu5 receptor surface expression (Lee et al., 2008). 
      2. DHPG-LTD doesn't require:
         1. PKC
         2. PKA
      3. synaptically-induced LTD
         1. PKC-dependent in some circumstances

            Annotations:

            • synaptically induced LTD in the CA1 may be dependent on PKC activation under some (Bolshakov and Siegelbaum, 1994; Oliet et al., 1997) but not all (Moult et al., 2008) circumstances
            1. MAPK

               Annotations:

               • PKC may mediate a decrease in synaptic transmission by stimulating MAPK cascades (Ferraguti et al., 1999), activating phospholipase D (Boss and Conn, 1992;Pellegrini-Giampietro et al., 1996) or phospholipase A2 (PLA2) (Aramori and Nakanishi, 1992), or modulating cation channel activity (Sharon et al., 1997).
               1. phospholipase D
               2. phospholipase A2
               3. modulating cation channel activity
      4. PTK

         Annotations:

         • It has been suggested that DHPG-induced LTD is blocked by PTK inhibitors in the dentate gyrus (Camodeca et al., 1999) but this finding was not replicated in area CA1 (Moult et al., 2002), suggesting the possibility of regional differences.
         1. in DG but not CA1
3. G-Proteins and Scaffolding and Regulatory Proteins

   Annotations:

   • mGluRs function as G-protein-coupled receptors in that agonist-induced or constitutive receptor activity leads to G-protein activation by promoting the exchange of GTP to GDP (Hermans and Challiss, 2001). This results in modulation of receptor-protein interactions and activation of distinct second messenger cascades.
   1. Group I- PLC- DAG & IP3
      1. DAG>> PKC
      2. IP3>> Ca
   2. Gαq subunit
      1. Gαq KO mice

         Annotations:

         • Both synaptically induced LTD and DHPG-LTD in the CA1 region were prevented in Gαq KO mice (Kleppisch et al., 2001).
         1. block agonist/ synaptically induced LTD
   3. scaffolding proteins
      1. Homer- Couple mGlu5 to ERK cascade

         Annotations:

         • Homer 1b/c may couple mGlu5 receptor activation to the Ca2+-independent ERK signaling cascade (Mao et al., 2005). This may involve association of mGlu5 receptor with the epidermal growth factor receptor tyrosine kinase (Peavy et al., 2001) and activation of Src nonreceptor tyrosine kinases (Luttrell et al., 1997).
         1. Evidence

            Annotations:

            • mGluR-LTD in the CA1 of the hippocampus is dependent on mGluR C-terminal interactions with Homer (Ronesi and Huber, 2008).
      2. caveolin-1, an adaptor protein

         Annotations:

         • associates with lipid rafts and the main protein of caveolae, binds to and colocalizes with group I mGluRs (Francesconi et al., 2009b).
         1. mGlu1/5 internalization

            Annotations:

            • The interaction with caveolin-1 affects the rate of constitutive mGlu1/5 internalization, thereby regulating the level of receptor expression at the cell surface
         2. MAPK- ERK

            Annotations:

            • association with caveolin-1 regulates mGluR-dependent phosphorylation/activation of ERK-MAPK (Francesconi et al., 2009b), which is required for mGluR-LTD in the hippocampus (Gallagher et al., 2004).