Activity-dependent modulation of the BDNF receptor TrkB: mechanisms and implications

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Activity-dependent modulation of the BDNF receptor TrkB: mechanisms and implications Guhan Nagappan and Bai Lu Section on Neural Development and Plasticity, National Institute of Child Health and Human Development, and Gene, Cognition and Psychosis Program (GCAP), National Institutes of Health, 35 Lincoln Drive, MSC 3714, Bethesda, MD 20892-4480, USA

Although brain-derived neurotrophic factor (BDNF) has emerged as a key regulator of activity-dependent synaptic plasticity, a conceptually challenging question is how this diffusible molecule achieves local and synapsespecific modulation. One hypothesis is that neuronal activity enhances BDNF signaling by selectively modulating TrkB receptors at active neurons or synapses without affecting receptors on neighboring, less-active ones. Growing evidence suggests that neuronal activity facilitates cell-surface expression of TrkB. BDNF secreted from active synapses and neurons recruits TrkB from extrasynaptic sites into lipid rafts, microdomains of membrane that are enriched at synapses. Postsynaptic rises in cAMP concentrations facilitate translocation of TrkB into the postsynaptic density. Finally, neuronal activity promotes BDNF-induced TrkB endocytosis, a signaling event important for many long-term BDNF functions. These mechanisms could collectively underlie synapsespecific regulation by BDNF. Introduction Activity-dependent modification of synapses, also known as synaptic plasticity, is a powerful means by which the brain builds neuronal circuits during development and controls cognitive functions and complex behaviors in the adult [1,2]. A salient feature of synaptic plasticity is ‘input-specificity’ or ‘synapse-specificity’, whereby structural and functional modification occurs only at synapses that experience changes in activity. Take hippocampal long-term potentiation (LTP), a cellular model for learning and memory, as an example [1]. When two sets of Schaffer collateral afferents converging on the same postsynaptic CA1 neurons are monitored, the enhancement of synaptic strength at one set of afferents does not extend to or interfere with the other set of afferents. Cellular and molecular mechanisms underlying activity-dependent and synapse-specific modulation has become a hot area of research in the recent years. Neurotrophins are a family of secreted proteins that have emerged as important regulators of synaptic plasticity (for reviews, see [3–6]). Among them, brain-derived neurotrophic factor (BDNF) is the most extensively Corresponding author: Lu, B. ([email protected]). Available online 28 July 2005

studied, and has been shown to modulate the development and function of synapses in various systems, ranging from the neuromuscular junction to the cortex [7]. In the hippocampus, substantial evidence suggests that BDNF facilitates both early-phase and late-phase LTP (E-LTP and L-LTP) [8–11]. Experiments using gene-knockout mice have shown that BDNF is required for LTP [12,13]. Application of BDNF enhanced LTP in the neonatal hippocampus, where endogenous BDNF levels are low [14]. Pairing a weak burst of synaptic stimulation with brief dendritic BDNF application caused immediate and robust induction of LTP [15]. Although acute application of BDNF has been reported to enhance synaptic transmission [16], studies from many laboratories have demonstrated that BDNF alone does not potentiate basal synaptic transmission at CA1 synapses [13,14,17–19]. Taken together, these results suggest that BDNF is a modulatory rather than an instructive factor for hippocampal LTP. Remarkably, neuronal activity often influences the effectiveness of BDNF. For example, BDNF-induced synaptic potentiation at the neuromuscular synapse was greatly enhanced when presynaptic terminals were mildly depolarized [20]. Elevated neuronal activity and Ca2C influx through NMDA-type glutamate receptors were required for BDNF regulation of dendritic arborization in the developing visual cortex [21]. In the hippocampus, BDNF regulation of synaptic responses to repetitive stimulation occurred only when presynaptic neurons were stimulated at high frequency (O50 Hz) [18]. Moreover, although BDNF alone was ineffective, pairing BDNF application with weak presynaptic stimulation induced LTP effectively [14,15]. These experiments suggest that synaptic potentiation induced by BDNF is ‘preferential’ or ‘selective’ for active synapses. There is also direct evidence that BDNF acts in a synapse-specific manner. Application of BDNF to cultured hippocampal neurons preferentially potentiated immature synapses (which have a lower release probability), with relatively little effect on mature synapses [22,23]. In a culture triplet system, in which an excitatory neuron innervates two postsynaptic target neurons, one glutamatergic and the other GABAergic, BDNF selectively potentiated the glutamatergic but not the GABAergic synapse [24]. In parallel, when two sets of Schaffer

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collateral–CA1 synapses in the same hippocampal slice were monitored simultaneously, BDNF treatment potentiated the tetanized pathway without affecting the synaptic efficacy of the untetanized pathway [18]. These results indicate that the effects of BDNF are spatially restricted and selective for certain synapses. More importantly, how can diffusible BDNF distinguish between active and inactive neurons and synapses? A plausible mechanism would be local synthesis and/or secretion of BDNF at the active synapse. This has been discussed in several recent reviews [3,9,25]. Alternatively, active synapses might respond better to BDNF than less active ones, and this could be achieved by activitydependent control of BDNF signaling. BDNF signaling is mediated by two different classes of receptors: the p75 neurotrophin receptor (p75NTR) and TrkB receptor tyrosine kinase [26,27]; so far, virtually all the synaptic effects of BDNF are attributed to TrkB. BDNF binding to TrkB triggers autophosphorylation of tyrosine residue in its intracellular domain, leading to activation of one of more of the three major signaling pathways involving mitogenactivated protein kinase (MAPK), phosphatidylinositol 3-kinase (PI3K) and phospholipase Cg (PLC-g). In this review, we address how neuronal activity regulates TrkB trafficking and signaling, and how these mechanisms could contribute to synapse-specific modulation by BDNF. We describe mechanisms by which neuronal activity regulates trkB transcription, transport and translation. We also discuss evidence for activity-dependent modulation of TrkB trafficking, including ligand-independent TrkB insertion and ligand-dependent endocytosis of TrkB. Neuronal activity also induces the secretion of BDNF, and an increase in the intracellular cAMP concentration. Recent experiments demonstrate that BDNF itself could induce lateral movement of TrkB from non-raft regions into lipid rafts [28], a component of the plasma membrane enriched at synapses. We also discuss how a postsynaptic rise in cAMP levels could facilitate translocation of TrkB into postsynaptic density and so gates TrkB tyrosine kinase activity [29]. Axonal transport of TrkB (either anterograde or retrograde) [30,31] has not been shown to be regulated by neuronal activity and therefore will not be discussed here. We believe that these recent studies could provide important insights into how synapse-specific modulation could be achieved by diffusible neurotrophins such as BDNF. Activity-dependent synthesis and transport of trkB mRNA At the whole-cell level, activity-dependent expression of trkB offers a potential mechanism for how more active neurons (not synapses) might respond better to afferent stimulation (Figure 1). It is well documented that elevated neuronal activity, such as in a limbic seizure, could enhance trkB gene expression [32–36]. More importantly, the level of trkB mRNA is significantly increased by LTP-inducing tetanic stimulation, with little or no effect on transcripts of other Trk receptors [37,38]. Whether weaker stimuli such as those used in spike-timing LTP [39] or weak tetanus alter TrkB expression remains to be studied. In addition to the catalytically active full-length www.sciencedirect.com

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TrkB isoform, alternative splicing of the primary transcript produces different isoforms of truncated TrkB (T1 and T2 in rat; T1 and T-shc in humans) that lack the intracellular kinase domain [40–42]. T1 is predominantly expressed in the brain [40] and can act as a dominantnegative inhibitor of BDNF signaling by forming heterodimers with the full-length TrkB [43–46]. Because T1 can compete for the ligand of TrkB and also alter its function, the effect of BDNF on synapses could be limited by expressing T1 on the extrasynaptic surface of adult neurons. Moreover, high-level expression of T1 in non-neuronal cells and/or in pathological conditions (e.g. injury or seizure) could limit the diffusion of BDNF, and this has been suggested to prevent axon regeneration at the site of injury [47–51]. Thus, controlling T1 expression could also contribute to synapse-specific regulation by locally secreted BDNF [3]. The trkB gene can be transcribed from alternate promoters, P1 and P2 [52] (Figure 1). P2 is located in an intron of the P1-driven transcript, resulting in an mRNA with different 5 0 -untranslated region (5 0 -UTR). In culture, depolarization of cortical neurons enhanced expression of full-length trkB mRNA [53]. This was attributed to Ca2C entry through voltage-gated Ca2C channels but not NMDA receptors, leading to activation of Ca2C-response elements (CREs) in P1 and P2. Neurons transiently transfected with trkB promoter–luciferase constructs showed that Ca2C activated P2 but inhibited the upstream P1. As will be discussed later in this review, it will be interesting to determine whether the P1-derived or P2derived 5 0 -UTR is responsible for selective (through RNAbinding proteins in RNA granules) and activity-dependent transport of trkB mRNA into the dendrites of hippocampal neurons. A more relevant mechanism is the regulation of dendritic trafficking and local translation of trkB mRNA (Figure 1). Tongiorgi and colleagues had demonstrated that neuronal activity promotes the translocation of trkB mRNA into dendrites both in vitro and in vivo [54,55]. In cultured hippocampal neurons under resting conditions, trkB mRNA labeling covers the proximal 30% of total dendritic length. Treatment with 10 mM KCl (for 3 h), which elicited mild depolarization and sustained neuronal firing, extended the labeling of trkB mRNA to 68% of the dendritic length [54]. Because activity also enhances trkB expression, one might argue that the dendritic increase in trkB mRNA was simply due to diffusion of trkB mRNA from cell body as a consequence of elevated gene expression. This is not the case because inhibition of transcription by actinomycinD did not block depolarization-induced dendritic increase in trkB mRNA. Such an increase required firing of action potentials, excitatory synaptic transmission and Ca2C influx. A short pulse (1 min) of exogenous BDNF itself, in the absence of depolarization, could also induce dendritic accumulation of trkB mRNA, suggesting that activity-dependent local secretion of BDNF could contribute to dendritic trafficking of trkB mRNA [56]. Interestingly, expression of both trkB mRNA and TrkB protein increased in the distal dendrites upon depolarization, although with different kinetics. Remarkably, although a short pulse of 10 mM KC could elicit an

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Figure 1. Activity-dependent expression and trafficking of trkB mRNA. (a) Under relatively quiet conditions (i.e. with few action potentials), trkB transcription is initiated from promoter P1 and the transcripts are transported to proximal regions of the primary dendrites. (b) Neuronal activity and Ca2C influx (i.e. when there are many action potentials) induce CREB-mediated promoter-P2-dependent trkB transcription, and the transcripts might be actively transported by (as-yet unidentified) RNA-binding proteins (red circles) to the distal regions of active dendritic branches.

increase in dendritic TrkB protein levels in the distal regions within 10 min, a significant quantitative increase in trkB mRNA levels was evident only after 3 h [54]. The increase in dendritic TrkB protein is not due to activitydependent increase in TrkB synthesis and transport from the cell body, because the KC-induced increase in dendritic TrkB protein levels still occurred when the dendritic transport was blocked. Taken together, these results suggest that neuronal activity also stimulates local dendritic translation of trkB mRNA, levels of which are elevated as a consequence of dendritic trafficking of trkB mRNA. The increase in dendritic trkB mRNA and its translation could both contribute to an elevated response to BDNF. Activity-dependent insertion of TrkB into the plasma membrane Compared with the increase in trkB mRNA transport and local translation, a faster and perhaps a more direct way to ensure preferential regulation of active synapses by BDNF is to increase local TrkB receptor insertion into active synapses (Figure 2a). Depolarization induced by high KC concentrations increased the levels of TrkB on the surface of retinal ganglion cells and spinal neurons [57]. Du et al. provided direct evidence for TrkB insertion at the surface of hippocampal neurons in response to www.sciencedirect.com

physiologically relevant stimuli [58]. Field tetanic stimulation, but not simple depolarization or low-frequency stimulation, markedly increased the number of surface TrkB receptors, as demonstrated by an increase in both surface bound 125I-BDNF and surface biotinylated TrkB. Such an increase was rapid (!30 min) and required no protein synthesis, but depended on Ca2C influx and activation of Ca2C/calmodulin-dependent kinase II (CaMKII). Surface immunofluorescence staining confirmed that the electrical stimulation facilitated the movement of TrkB from the intracellular pool to the cell surface, particularly in dendrites. These results suggest that the mechanism involved in activity-dependent surface TrkB expression is similar to that used by AMPA receptor insertion into the postsynaptic membrane during hippocampal LTP [59]. Inhibition of excitatory synaptic transmission blocked the effect of electric stimulation on TrkB surface expression, suggesting that active synaptic transmission facilitates insertion of TrkB at the synapse. Preferential elevation of TrkB levels at highly active synapses could ensure local action of BDNF without affecting nearby less-active synapses. In addition to its direct effect on TrkB insertion, neuronal activity could also regulate the surface expression of TrkB through local secretion of BDNF. Acute exposure of hippocampal neurons to BDNF rapidly increased TrkB

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Figure 2. Regulation of TrkB trafficking by neuronal and/or synaptic activity. (a) Activity-dependent and BDNF-independent insertion of TrkB at the cell surface. Neuronal and/or synaptic activity induces Ca2C influx through NMDA receptors and Ca2C channels even in the absence of BDNF. Subsequent activation of CaMKII facilitates fusion of TrkB-containing vesicles to the cell membranes. (b) BDNF-induced recruitment of TrkB into lipid rafts. Neuronal and/or synaptic activity could trigger local secretion of BDNF. Binding of ligands (BDNF or NT4/5) activates TrkB receptor tyrosine kinase (indicated by red stars), which is crucial for lateral movement of TrkB from non-raft to raft regions of plasma membranes. BDNF-induced recruitment of TrkB into rafts could serve as a mechanism to ensure synapse-specific regulation by BDNF. (c) cAMP-induced translocation of TrkB into the postsynaptic density and gating of TrkB signaling by cAMP. Neuronal and/or synaptic activity also triggers an increase in [cAMP]i, which could contribute synapse-specific modulation in two ways: (i) by facilitating movement of TrkB into spines and/or the postsynaptic density (green arrow) or (ii) by serving as a gate for the signaling of TrkB, which could happen either inside (right-hand blue arrow) or outside (left-hand blue arrow) synapses. (d) Activity-dependent endocytosis of the BDNF–TrkB complex. Ca2C influx through NMDA receptors and Ca2C channels, as a consequence of local synaptic activity, could enhance TrkB receptor tyrosine kinase activity, which in turn facilitates ligand-induced internalization of TrkB. Endocytosed TrkB remains active (ligand-bound) and, together with associated signaling molecules (MAPK, PI3K and PLCg), forms signaling endosomes.

surface expression (within 15 s), whereas chronic treatment with BDNF (for 3 h) led to decreased surface TrkB levels [46]. Surprisingly, BDNF-induced downregulation of TrkB was due to proteasome-mediated degradation, rather than lysosome-mediated removal after endocytosis [60]. Further work is necessary to determine the physiological significance and molecular mechanisms underlying surface TrkB regulation by BDNF. TrkB translocation into lipid rafts on cell membrane: ligand-dependent regulation An alternative mechanism to constrain BDNF modulation to highly active synapses is through the lateral movement of TrkB from extrasynaptic sites into the synapse, which is enriched in lipid rafts (cholesterol and sphingolipid-rich microdomains), in response to BDNF (Figure 2b) [61,62]. Lipid rafts make a specialized signaling platform, with higher concentrations and often different signaling www.sciencedirect.com

molecules, as compared with non-raft regions. Translocation of TrkB into rafts, therefore, makes it more effective in transducing BDNF signaling. Moreover, lipid rafts are enriched in axonal terminals [63] and dendrites [64]. Many presynaptic and postsynaptic proteins are enriched in the raft fraction [64–67], and there appear to be more rafts in synaptosomal membranes [68]. Thus, it is possible that translocation of TrkB into lipid rafts results in better BDNF signaling at synapses. Suzuki et al. recently demonstrated that acute exposure to BDNF rapidly (!30 min) and selectively translocated full-length TrkB from non-rafts to lipid rafts in cultured cortical neurons [28]. Unlike c-Ret, whose recruitment into rafts upon ligand induction is independent of its tyrosine kinase activity [69,70], TrkB tyrosine kinase activity is essential for the BDNF-dependent TrkB translocation in to lipid rafts. TrkB did not carry its associated signaling molecules into lipid rafts during

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translocation. Rather, it selectively activated the MAPK pathway when it entered the raft domains, without affecting the PI3K–Akt pathway. Thus, the lipid raft seems to compartmentalize signaling molecules on the plasma membrane, which allows them to interact with each other and prevents interactions with proteins that are excluded from the rafts. What is the functional consequence of BDNF-induced TrkB translocation into lipid rafts? Suzuki et al. used methyl-b-cyclodextrin (MCD), which binds to cholesterol and depletes it from the plasma membrane, to block BDNF-induced TrkB translocation [28]. They showed that disruption of TrkB translocation into lipid rafts abolished the acute potentiating effect of BDNF on evoked synaptic transmission in culture, and prevented enhancement of the synaptic response to tetanus in hippocampal slices. Interestingly, TrkB translocation into rafts was not important for cortical neuron survival. It is possible that TrkB could signal locally at the plasma membrane to alter synaptic function, but signal over a long distance via retrograde transport to mediate cell survival. In cultured spinal neurons, treatment with MCD selectively blocked BDNF-induced growth-cone turning, without affecting axonal elongation [63]. Thus, recruitment of TrkB into rafts elicits specific effects that are relevant for synaptic function. Trafficking to synapses: regulation by cAMP In addition to inducing BDNF secretion, neuroelectric activity also increases the local intracellular concentration of cAMP ([cAMP]i), which might contribute to synapse-specific actions of BDNF. Substantial evidence indicates that cAMP is crucial in BDNF regulation of synapses. For example, purified retinal ganglion cells (RGCs) grow in BDNF-containing medium only if their [cAMP]i is simultaneously increased [71,72]. However, increasing [cAMP]i alone does not promote RGC survival. Subsequent studies demonstrated that [cAMP]i elevation enhanced the responsiveness of RGCs to BDNF by increasing the surface expression of TrkB [57]. Longterm treatment of hippocampal slices with BDNF elicited a delayed, protein-synthesis-dependent increase in the level of synaptotagmin, a vesicular membrane protein proposed to be the Ca2C sensor for synaptic vesicle fusion. Although the BDNF-induced increase of synaptotagmin levels was blocked by inhibiting the cAMP–protein kinase A (PKA) pathway, BDNF did not activate PKA, and application of a PKA activator did not mimic the effect of BDNF. Thus, cAMP is not a downstream effector in the BDNF-mediated signaling cascade, but instead is permissive for the BDNF effect [73]. Experiments using Xenopus nerve–muscle co-cultures provided some important new insights in to the relationship between BDNF and PKA signaling mechanisms [20]. Application of BDNF induced a rapid potentiation of transmission at neuromuscular synapses. Such potentiation exhibited the following features: (i) inhibitors of cAMP signaling blocked potentiation induced by high doses of BDNF; (ii) activators of cAMP signaling enhanced the potentiating effects of low-dose BDNF; and (iii) cAMP analogs alone did not mimic the BDNF effects. Based on these experiments, www.sciencedirect.com

cAMP was proposed to act as a ‘gate’ that enables BDNF to achieve its synaptic effects. Similar ‘cAMP gating’ features were observed in the long-term regulation of dendritic spine formation in hippocampal neurons, without affecting the acute effect of modulating hippocampal plasticity mediated by BDNF [29]. Thus, cAMP gating of BDNF regulation is specific. What molecular mechanism(s) underlie the cAMP gating of BDNF function? A recent report by Ji et al. demonstrated that cAMP modulates TrkB signaling in hippocampal neurons through two distinct mechanisms (Figure 2c) [29]. First, cAMP regulates BDNF-induced TrkB tyrosine phosphorylation, exhibiting all three characteristic features for cAMP gating. BDNF-induced TrkB phosphorylation was attenuated by inhibitors of cAMP signaling and was potentiated by cAMP analogs, but was immune to activators of the cAMP–PKA pathway. These results suggest that BDNF–TrkB signaling might be selectively enhanced in active neurons or synapses with elevated [cAMP]i. Second, cAMP facilitated the movement of TrkB into the postsynaptic density of hippocampal neurons. Both double staining and co-immunoprecipitation experiments showed that TrkB and the postsynaptic density protein PSD-95 physically interact and colocalize in the dendritic spines after 15 min treatment with activators of the cAMP–PKA pathway. Thus, this pathway might selectively enhance the translocation of TrkB into PSD-95-containing spines and/or synapses by facilitating the association of TrkB with the PSD-95 complex. Activity-dependent and tyrosine-kinase-dependent regulation of TrkB endocytosis In addition to regulating TrkB insertion, neuronal activity also enhances endocytosis of the BDNF–TrkB complex (Figure 2d). For most growth factors, endocytosis of their receptors terminates growth-factor signaling [74]. However, endocytosis of activated Trk receptors is an important step in some biological functions of neurotrophins [75–78]. The ligand–receptor complex has been shown to be internalized through clathrin-mediated endocytosis [79] and more recently through clathrin-independent, pincher-mediated macropinocytosis with membrane ruffles [80,81]. Instead of going to late endosomes or lysosomes, the receptor complex goes to a specialized vesicular compartment called the signaling endosome [79,82,83]. According to studies based on the nerve growth factor (NGF)–TrkA system, internalized TrkA remains tyrosine phosphorylated and active, with its extracellular domain bound to NGF inside signaling endosomes and its intracellular domain tightly associated with several signaling proteins in the PLC-g, PI3K and MAPK pathways in the cytoplasm of responsive neurons [83,84]. There is also evidence suggesting that Trk receptors in signaling endosomes activate signaling events that are different from those activated on the cell surface [85,86]. Similar mechanisms might also operate in BDNF–TrkB signaling. If endocytosis is an important step in TrkB signaling, facilitation of TrkB endocytosis by activity could ensure better response to BDNF in active synapses (Figure 2b). Neuroelectric activity has recently been shown to

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facilitate ligand-induced endocytosis of TrkB in hippocampal neurons [87]. This was demonstrated using three complementary approaches. First, BDNF receptor endocytosis was quantitatively measured by counting I125-BDNF within cultured neurons after acid wash to remove surface-bound I125-BDNF. Second, to distinguish whether activity affects TrkB or p75NTR (because both bind BDNF), biotinylated endocytosed surface proteins were precipitated by avidin, followed by western blot using an antibody to TrkB. Finally, TrkB endocytosed following application of fluorescent-tagged BDNF was visualized in different intracellular compartments. Unlike activity-dependent TrkB surface insertion, which is BDNF-independent and stimulation-frequencydependent, TrkB endocytosis depended on the ligand BDNF but not on stimulation frequency. Ca2C influx through NMDA receptors and Ca2C channels was also required. Because electric stimulation enhanced TrkB endocytosis triggered by exogenous BDNF in neurons derived from BDNFK/K mice, the activity-dependent TrkB endocytosis was not due to enhanced secretion of endogenous BDNF induced by electrical stimulation. An unexpected finding by Du et al. was that electrical stimulation also enhanced TrkB tyrosine kinase activity [87]. Neuronal activity has been shown to rapidly activate TrkB tyrosine kinase, but this was interpreted as a consequence of activity-dependent secretion of BDNF [88–90]. Using cultured hippocampal neurons, Du et al. demonstrated that electrical stimulation and Ca2C influx enhanced the BDNF-induced, TrkB-mediated phosphorylation. Two pieces of evidence suggest that the activitydependent enhancement of TrkB tyrosine kinase in the hippocampal neurons was not due to elevated secretion of endogenous BDNF. (i) In absence of exogenous BDNF, electrical stimulation did not activate TrkB phosphorylation, suggesting that the amount of endogenous BDNF secreted by electrical stimulation was minimal. (ii) Electrical stimulation further enhanced TrkB tyrosine phosphorylation induced by saturating concentrations of BDNF. Therefore, neuroelectric activity directly modulates TrkB tyrosine kinase function. What is the relationship between the effects of neuroelectric activity on TrkB endocytosis and on TrkB tyrosine kinase activity? Inhibition of TrkB endocytosis did not alter activity-induced increases in TrkB tyrosine phosphorylation. By contrast, inhibition of TrkB tyrosine kinase, either by the Trk kinase inhibitor k252a or dominant-negative TrkB-T1, prevented the potentiating effect of electrical stimulation on BDNF-induced TrkB endocytosis. These results suggest that activity-dependent TrkB endocytosis is mediated by the receptor tyrosine kinase itself. The activity-dependent and Ca2C-dependent modulation of TrkB tyrosine kinase and its endocytosis provides an alternative and perhaps more physiologically relevant mechanism by which preferential regulation of active synapses could be achieved. TrkB: a synaptic tag? Activity-dependent control of TrkB receptor raises the possibility that it could function as a synaptic tag. According to the ‘synaptic tagging’ hypothesis proposed www.sciencedirect.com

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by Frey and Morris, synapse-specificity of L-LTP in the hippocampus is achieved by the interaction of plasticity related proteins, which are synthesized in the cell body or dendrites, with the synaptic tags that are generated locally at the stimulated synapses [91]. The findings discussed in this review posit TrkB as a possible candidate for synaptic tag. TrkB activation is reversible (with a halflife of 1–2 h), is spatially restricted (signaling enhanced only at active synapses), and could interact with BDNF transcribed from the nucleus to produce long-lasting synapse-specific strengthening. Moreover, it has been shown that BDNF can convert E-LTP to L-LTP, and rescue L-LTP deficits in the presence of protein-synthesis inhibitors [11]. Together, this evidence suggests that TrkB could act as a synaptic tag. Further tests of this hypothesis represent an exciting area for future research. Concluding remarks A key issue in the regulation of synaptic plasticity by BDNF is how to achieve activity-dependent and synapsespecific modulation, given that BDNF itself is diffusible. Recent studies have revealed several novel mechanisms that could constrain BDNF modulation to active neurons and/or synapses by controlling cellular responsiveness to neurotrophins – that is, local and synapse-specific regulation of TrkB signaling. Activity-dependent regulation of trkB transcription has been well documented, but such a mechanism alone is unlikely to ensure synapse specificity because transcription occurs in the neuronal cell body. Although there is substantial evidence for activity-dependent trafficking of trkB mRNA into dendrites, selective transport of trkB mRNA to active synapses has not been demonstrated. Trafficking of trkB mRNA into dendrites implies dendritic translation of trkB mRNA, which opens the possibility for local synthesis of TrkB protein right at the active synapses. Two additional mechanisms have been demonstrated in cultured hippocampal neurons: activity-dependent insertion of TrkB at the neuronal surface, and activity-dependent control of BDNF–TrkB complex endocytosis. Both the mechanisms should lead to activity-dependent enhancement of TrkB signaling. Although these findings are intriguing, it remains to be established whether the insertion occurs at synapses or extrasynaptically. It is also important to demonstrate whether activity controls TrkB insertion and/or endocytosis at presynaptic or postsynaptic sites. Additional experiments are necessary to confirm that these mechanisms are actually operative in vivo. Neuronal and synaptic activity could induce local secretion of BDNF, which in turn could trigger TrkB translocation into lipid rafts. It is tempting to speculate that local secretion of BDNF in response to elevated synaptic activity could recruit TrkB into presynaptic or postsynaptic membranes (or both), leading to the selective modulation of active synapses. In this context, activity-dependent secretion of BDNF, together with the increase in TrkB insertion and/or endocytosis, could amplify the modulatory effects of BDNF on an active synapse. Conceptually, TrkB–BDNF signaling could serve as a ‘coincidence detector’, similar to NMDA receptors, although on a much slower timescale (minutes). Neuronal and/or synaptic activity could also elicit an

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increase in [cAMP]i locally at synapses. cAMP could facilitate TrkB signaling through gating of its tyrosine kinase activity. More importantly, cAMP could induce the translocation of TrkB into the postsynaptic density. These represent exciting new mechanisms that could restrict BDNF actions to active synapses. Further studies are necessary to test whether these mechanisms are relevant to the regulation of synaptic plasticity by BDNF in vivo.

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