Aplicación del procedimiento para proponer las competencias laborales

1.7.4.1 The role of Arc in memory and synaptic plasticity

Arc has been shown to play a role in a range of different functions in the brain including, neurogenesis, depression, drug addiction, memory and synaptic plasticity (LTP, LTD and homeostatic scaling) (Bramham et al., 2010). The temporal regulation of Arc transcription and translation determines its function in the processes mentioned above. For example, Arc is required for LTP consolidation, i.e. transferring short term memories to long term memories, thus stabilising memories, which was elegantly shown by using Arc antisense and Arc KO mice. In in vivo studies using anaesthetised rats, an Arc antisense (AS) oligodeoxynucleotide was infused into the dentate gyrus which caused impairments in HFS-induced LTP maintenance but not induction (Messaoudi et al., 2007). The requirement for Arc during LTP consolidation but not induction was shown in behavioural training showing the importance of Arc for long-term memory formation. This was reflected

74 where mice needed Arc in order to form long- term spatial memory but not for short- term memory performance (Guzowski et al., 2000). This observation was demonstrated in Arc KO mice where their early phase of LTP was enhanced but late phase LTP was blocked in both the dentate gyrus in vivo and in the CA1 region of acute hippocampal slices (Plath et al., 2006). A possible explanation for the enhanced early LTP in the Arc KO animals might be that they have an altered metaplasticity (Abraham and Bear, 1996). This means that since the KO synapses can't consolidate prior to potentiation they are then constitutively in a more plastic state leading to an enhanced early LTP. The authors of the Plath et al., 2006 paper also claim that the enhanced early LTP could be a consequence of decreased activity-dependent AMPAR endocytosis. However LTP has been largely defined as an increase in AMPAR insertion rather than a reduction in endocyotis.

The later stages of LTP involves memory consolidation which may explain why the KO mice have severe deficits in spatial memory tasks. Interestingly these transgenic mice also have reduced LFS induced LTD at the Schaffer collateral/CA1 pyramidal cells synapses (Plath et al., 2006). The Arc-dependent mechanism involved during LTD was shown to involve its role in endocytosis of AMPAR. A study was carried out in hippocampal slices from WT and Arc KO mice where DHPG-induced LTD failed to decrease surface GluA1 expression thus preventing LTD in Arc KO mice (Park et al., 2008). Under homeostatic scaling conditions Arc KO hippocampal neurones were unable to scale either their mEPSC amplitude or GluA1 surface expression in the presence of either bicuculline or TTX (Shepherd et al., 2006).

Recent findings have shown that Arc can be post-translationally modified by a process known as SUMOylation. The small ubiquitin-like modifier (SUMO) protein is known to determine subcellular localisation of proteins. Arc has two SUMOylation sites that direct its expression to the dendrites and the cytoskeleton (Nair et al., 2009). LTP in the dentate gyrus of anesthetised rats induced an upregulation of SUMOylated Arc in the cytosol (Bramham et al., 2010). More recently SUMOylated Arc has been implicated in playing a role during TTX-induced homeostatic scaling (Craig and Henley, 2012). A construct was designed which mutated the two lysine residues which are SUMOylated in Arc. These neurones expressing the Arc double mutation were unable to up-scale their AMPAR expression in response to TTX when compared to control cells (Craig et al., 2012).

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1.7.4.2 BDNF-dependent regulation of Arc

BDNF-induced LTP using BDNF infusion in the dentate gyrus results in an upregulation of Arc mRNA and protein, where in situ hybridisation showed that Arc transcripts were rapidly transported to granule cell dendrites (Ying et al., 2002). This mechanism was also shown to be MEK-ERK dependent as the U0126 MEK inhibitor blocked the upregulation of Arc upon BDNF-LTP. Interestingly this requirement for Arc transcription for synaptic strengthening was shown to be important during a specific time frame. When the transcription inhibitor, actinomycin D, was applied in vivo in the dentate gyrus, 1 hour or immediately before BDNF stimulation it blocked BDNF-LTP. However when actinomycin D was applied 2 hours after BDNF stimulation there was no effect (Messaoudi et al., 2002).

The role of Arc during LTP consolidation not only requires sustained Arc protein translation but Arc has also been shown to regulate actin dynamics. This was shown in a similar study where the inhibition of LTP consolidation using the Arc antisense was associated with rapid dephosphorylation of hyperphosphorylated cofilin (Messaoudi et al., 2007). Cofilin regulates F-actin dynamics where, when it is in a phosphorylated state, actin polymerisation is promoted. Therefore the rapid dephosphorylation is associated with the loss of F-actin. Furthermore the effect of Arc anti-dense (AS) on reversing LTP was blocked when an F-actin stabilising drug, jasplakinolide was applied to the dentate gyrus where LTP was maintained.

1.7.4.3 Arc and dendritic spine morphology

Arc dependent changes in actin cytoskeleton and AMPAR trafficking can influence structural changes in dendritic spines. Overexpressed Arc in hippocampal neurones increases the proportion of thin ‘learning’ spines containing reduced GluA1 expression (Peebles et al., 2010). By increasing the number of ‘learning’ spines the neurones are able to respond to changes in activity more readily. Arc has been implicated in regulating network excitability due to homeostatic regulation of AMPAR trafficking and spine morphology (Lyford et al., 1995). The network excitability was addressed by looking at the susceptibility of Arc KO neurones to convulsant drugs. The data showed that the loss of Arc expression in vivo lead to aberrant neuronal overexcitation. The loss of the homeostatic negative feedback could explain the epileptic-like state and the associated spine loss (Shepherd et al., 2006; Peebles et al., 2010).

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