Elhoucine Messaoudi

Sustained Arc/Arg3.1 Synthesis Controls Long-Term Potentiation Consolidation through Regulation of Local Actin Polymerization in the Dentate Gyrus In Vivo

New gene expression is necessary for long-term potentiation (LTP) consolidation, yet roles for specific activity-induced mRNAs have not been defined. Here we probed the dynamic function of activity-induced Arc (activity-regulated cytoskeletal-associated protein)/Arg3.1 (activity-regulated gene 3.1 protein homolog) mRNA using brief, local infusions of antisense (AS) oligodeoxynucleotides at multiple time points during dentate gyrus LTP in vivo. Surprisingly, early Arc synthesis is necessary for early expression of LTP, whereas sustained synthesis is required to generate stably modified synapses. AS application 2 h after LTP induction results in a rapid and permanent reversal of LTP. This reversal is associated with rapid knockdown of upregulated Arc, dephosphorylation of actin depolymerization factor/cofilin, and loss of nascent filamentous actin (F-actin) at synaptic sites. Infusion of the F-actin stabilizing drug jasplakinolide during LTP maintenance blocks the ability of AS to reverse LTP. These results couple activity-induced expression of Arc to expansion of the actin cytoskeleton underlying enduring LTP. Furthermore, Arc synthesis is required for both the induction and consolidation of LTP elicited by local BDNF infusion, thus identifying Arc as a key molecular effector of BDNF in synaptic plasticity.

Brain-Derived Neurotrophic Factor Triggers Transcription-Dependent, Late Phase Long-Term Potentiation In Vivo

Acute intrahippocampal infusion of brain-derived neurotrophic factor (BDNF) leads to long-term potentiation (BDNF-LTP) of synaptic transmission at medial perforant path→granule cell synapses in the rat dentate gyrus. Endogenous BDNF is implicated in the maintenance of high-frequency stimulation-induced LTP (HFS-LTP). However, the relationship between exogenous BDNF-LTP and HFS-LTP is unclear. First, we found that BDNF-LTP, like HFS-LTP, is associated with enhancement in both synaptic strength and granule cell excitability (EPSP–spike coupling). Second, treatment with a competitive NMDA receptor (NMDAR) antagonist blocked HFS-LTP but had no effect on the development or magnitude of BDNF-LTP. Thus, NMDAR activation is not required for the induction or expression of BDNF-LTP. Formation of stable, late phase HFS-LTP requires mRNA synthesis and is coupled to upregulation of the immediate early gene activity-regulated cytoskeleton-associated protein (Arc). Local infusion of the transcription inhibitor actinomycin D (ACD) 1 hr before or immediately before BDNF infusion inhibited BDNF-LTP and upregulation of Arc protein expression. ACD applied 2 hr after BDNF infusion had no effect, defining a critical time window of transcription-dependent synaptic strengthening. Finally, the functional role of BDNF-LTP was assessed in occlusion experiments with HFS-LTP. HFS-LTP was induced, and BDNF was infused at time points corresponding to early phase (1 hr) or late phase (4 hr) HFS-LTP. BDNF applied during the early phase led to normal BDNF-LTP. In contrast, BDNF-LTP was completely occluded during the late phase. The results strongly support a role for BDNF in triggering transcription-dependent, late phase LTP in the intact adult brain.

Brain-derived neurotrophic factor and control of synaptic consolidation in the adult brain

Interest in BDNF (brain-derived neurotrophic factor) as an activity-dependent modulator of neuronal structure and function in the adult brain has intensified in recent years. Localization of BDNF and its receptor tyrosine kinase TrkB (tropomyosin receptor kinase B) to glutamate synapses makes this system attractive as a dynamic, activity-dependent regulator of excitatory transmission and synaptic plasticity in the adult brain. Development of stable LTP (long-term potentiation) in response to high-frequency stimulation requires new gene expression and protein synthesis, a process referred to as synaptic consolidation. Several lines of evidence have implicated endogenous BDNF-TrkB signalling in synaptic consolidation. This mini-review emphasizes new insights into the molecular mechanisms underlying this process. The immediate early gene Arc (activity-regulated cytoskeleton-associated protein) is strongly induced and transported to dendritic processes after LTP induction in the dentate gyrus in live rats. Recent work suggests that sustained synthesis of Arc during a surprisingly protracted time-window is required for hyperphosphorylation of actin-depolymerizing factor/cofilin and local expansion of the actin cytoskeleton in vivo. Moreover, this process of Arc-dependent synaptic consolidation is activated in response to brief infusion of BDNF. Microarray expression profiling has also revealed a panel of BDNF-regulated genes that may co-operate with Arc during LTP maintenance. In addition to regulating gene expression, BDNF signalling modulates the fine localization and biochemical activation of the translation machinery. By modulating the spatial and temporal translation of newly induced (Arc) and constitutively expressed mRNA in neuronal dendrites, BDNF may effectively control the window of synaptic consolidation. These findings have implications for mechanisms of memory storage and mood control.

Identification of genes co‐upregulated with Arc during BDNF‐induced long‐term potentiation in adult rat dentate gyrus in vivo

Brain‐derived neurotrophic factor (BDNF) is a critical regulator of transcription‐dependent adaptive neuronal responses, such as long‐term potentiation (LTP). Brief infusion of BDNF into the dentate gyrus of adult anesthetized rats triggers stable LTP at medial perforant path‐granule synapses that is transcription‐dependent and requires induction of the immediate early gene Arc. Rather than acting alone, Arc is likely to be part of a larger BDNF‐induced transcriptional program. Here, we used cDNA microarray expression profiling to search for genes co‐upregulated with Arc 3 h after BDNF‐LTP induction. Of nine cDNAs encoding for known genes and up‐regulated more than four‐fold, we selected five genes, Narp, neuritin, ADP‐ribosylation factor‐like protein‐4 (ARL4L), TGF‐β‐induced immediate early gene‐1 (TIEG1) and CARP, for further validation. Real‐time PCR confirmed robust up‐regulation of these genes in an independent set of BDNF‐LTP experiments, whereas infusion of the control protein cytochrome C had no effect. In situ hybridization histochemistry further revealed up‐regulation of all five genes in somata of post‐synaptic granule cells following both BDNF‐LTP and high‐frequency stimulation‐induced LTP. While Arc synthesis is critical for local actin polymerization and stable LTP formation, several of the co‐upregulated genes have known functions in excitatory synaptogenesis, axon guidance and glutamate receptor clustering. These results provide novel insight into gene expression responses underlying BDNF‐induced synaptic consolidation in the adult brain in vivo.

BDNF-induced LTP in dentate gyrus is impaired with age: analysis of changes in cell signaling events

Brain-derived neurotrophic factor (BDNF) has emerged as a major regulator of synaptic plasticity in the adult brain and acute BDNF infusion has been shown to trigger long-term potentiation (BDNF-LTP) in adult rats. Here we compared the effects of acute BDNF infusion in young adult and aged anesthetized rats. In young rats, BDNF-LTP was accompanied by increased activation of the BDNF receptor TrkB, and extracellular signal-regulated kinase (ERK), as well as enhanced evoked release of glutamate in synaptosomes prepared from DG. In aged rats, both BDNF-LTP and the associated signaling were significantly impaired, while analysis of untreated hippocampal tissue from aged rats showed that activation of TrkB and ERK were decreased. In addition to effects in the DG, BDNF-LTP was accompanied by robust phosphorylation of the calcium/cAMP-responsive element binding protein (CREB) in tissue prepared from entorhinal cortex of both young and aged rats. These results suggest a cascade of presynaptic changes contributing to the expression of BDNF-induced LTP and show that BDNF-induced transduction mechanisms are attenuated with age.

Dual regulation of translation initiation and peptide chain elongation during BDNF-induced LTP in vivo : evidence for compartment-specific translation control

Protein synthesis underlying activity‐dependent synaptic plasticity is controlled at the level of mRNA translation. We examined the dynamics and spatial regulation of two key translation factors, eukaryotic initiation factor 4E (eIF4E) and elongation factor‐2 (eEF2), during long‐term potentiation (LTP) induced by local infusion of brain‐derived neurotrophic factor (BDNF) into the dentate gyrus of anesthetized rats. BDNF‐induced LTP led to rapid, transient phosphorylation of eIF4E and eEF2, and enhanced expression of eIF4E protein in dentate gyrus homogenates. Infusion of the extracellular signal‐regulated kinase (ERK) inhibitor U0126 blocked BDNF‐LTP and modulation of the translation factor activity and expression. Quantitative immunohistochemical analysis revealed enhanced staining of phospho‐eIF4E and total eIF4E in dentate granule cells. The in vitro synaptodendrosome preparation was used to isolate the synaptic effects of BDNF in the dentate gyrus. BDNF treatment of synaptodendrosomes elicited rapid, transient phosphorylation of eIF4E paralleled by enhanced expression of α‐calcium/calmodulin‐dependent protein kinase II. In contrast, BDNF had no effect on eEF2 phosphorylation state in synaptodendrosomes. The results demonstrate rapid ERK‐dependent regulation of the initiation and elongation steps of protein synthesis during BDNF‐LTP in vivo. Furthermore, the results suggest a compartment‐specific regulation in which initiation is selectively enhanced by BDNF at synapses, while both initiation and elongation are modulated at non‐synaptic sites.

Chronic fluoxetine induces region‐specific changes in translation factor eIF4E and eEF2 activity in the rat brain

The delayed therapeutic onset observed in response to chronic antidepressant drug treatment is little understood. While current theories emphasize effects on gene transcription, possible effects of antidepressant drugs on translation control pathways have not been explored. We examined the effect of the selective serotonergic reuptake inhibitor fluoxetine on regulation of two major determinants of mRNA translation, eukaryotic initiation factor 4E (eIF4E) and eukaryotic elongation factor 2 (eEF2). Chronic fluoxetine treatment induced hyperphosphorylation of eEF2 (Thr56) in prefrontal cortex, hippocampus and dentate gyrus of rats. By contrast, phosphorylation of eIF4E (Ser209) was observed specifically in the dentate gyrus. Acute fluoxetine treatment had no effect on translational factor activity. These findings suggest that region‐specific regulation of translation contributes to the delayed action of antidepressant drugs such as fluoxetine.

Selective survival and maturation of adult-born dentate granule cells expressing the immediate early gene Arc/Arg3.1.

Progenitor cells in the adult dentate gyrus provide a constant supply of neuronal precursors, yet only a small fraction of these cells survive and develop into mature dentate granule cells (DGCs). A major challenge of current research is thus to understand the stringent selection process that governs the maturation and functional integration of adult-born DGCs. In mature DGCs, high-frequency stimulation (HFS) of the perforant path input elicits robust expression of the immediate early gene Arc/Arg3.1, trafficking of its mRNA to dendrites, and local synthesis of the protein necessary for consolidation of long-term potentiation (LTP). Given the synaptic commitment inherent in LTP consolidation, we considered that HFS-evoked expression of Arc could be used to timemap the functional integration of newborn DGCs. Dividing cells were birthmarked by BrdU-labeling at 1, 7, 14, 21, or 28 days prior to induction of LTP and expression of Arc was examined by confocal microscopy. Contrary to expectation, LTP did not induce Arc expression in newborn cells at any age, suggesting they might be refractory to synaptically-evoked Arc expression for at least one month. Importantly, however, spontaneous expression of Arc was detected in BrdU-labeled cells and strongly associated with the survival and maturation of NeuN-positive DGCs. Moreover, Arc expression at the earliest ages (1 and 7 days), clearly precedes the formation of glutamatergic synapses on new neurons. These results suggest an unexpected early role for Arc in adult-born DGCs, distinct from its functions in LTP, LTD, and homeostatic synaptic plasticity.

BDNF as a Trigger for Transsynaptic Consolidation in the Adult Brain

The neurotrophin family of signaling proteins, including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and NT-4i5 are crucially involved in regulating the survival and differentiation of neuronal populations during development (Levi Montalcini, 1987; Davies, 1994; Lewin and Barde, 1996). In addition to these well-established functions in development, a large body of work suggests that neurotrophins continue to shape the structure and function of neuronal connections throughout life (Schnell et al., 1994; Thoenen, 1995; Bonhoeffer, 1996; Prakash et al., 1996; Cabelli et al., 1997; Alsina et al., 2001; Maffei, 2002; Bolanos and Nestler, 2004; Duman, 2004; Tuszynski and Blesch, 2004). While neurotrophins traditionally were thought to operate on a time scale of days and weeks, extremely rapid effects have now been demonstrated on ion channels, neurotransmitter release, axon pathfinding, gene expression and mRNA translation (Song and Poo, 1999; Desai et al., 1999; Schinder and Poo, 2000). It has nevertheless been difficult to pin down precise functions for specific neurotrophins in adulthood. One of the most contested areas is the contribution of neurotrophins to activity-dependent synaptic plasticity. In a series of recent advances several lines of evidence have converged to specficially implicate BDNF in long-term potentiation (LTP), the most widely studied form of synaptic plasticity in the adult brain. BDNF is uniquely positioned to regulate synaptic efficacy through bidirectional effects at the glutamate synapse. The complexity and versatility of BDNF signaling is reflected in the multiple roles of this neurotrophin not only in LTP, but also in modulation of longterm depression (LTD), various forms of short-term synaptic plasticity, and homeostatic regulation of intrinsic neuronal excitability (Desai et al., 1999; Sermasi et al., 2000; Asztely et al., 2000; Kumura et al., 2000; lkegaya et al., 2002; Jiang et al., 2003). In this chapter we will briefly review key evidence for permissive and instructive actions of BDNF in hippocampal LTP. We will further elaborate on new evidence suggesting that BDNF drives the formation of stable, protein synthesis-dependent LTPa process we refer to as synaptic consolidation. A working model for synaptic consolidation based on induction of the immediate early gene, ArciArg, and local enhancement of dendritic protein synthesis, is proposed.