Trends in Neurosciences
ReviewNucleocytoplasmic protein shuttling: the direct route in synapse-to-nucleus signaling
Introduction
Long-lasting changes in synaptic input and synapto-dendritic cytoarchitecture require gene transcription. These changes are brought about by synaptic activity, which triggers signaling pathways to the nucleus that control transcriptional regulation 1, 2. The requirements for nuclear signaling processes can be observed in studies of long-term potentiation, a synaptic adaptation that is believed to underlie memory consolidation in rodents [3]. The late phases of long-term potentiation or depression (LTP or LTD, respectively) seem to be dependent on gene expression, whereas early phases are not 4, 5. The classical view of synapse-to-nucleus communication mainly invokes the Ca2+ ion as the principal messenger for neuronal activity regulated gene expression (for review, see Ref. [6]). Synaptic L-type Ca2+ channels and N-methyl-d-aspartic acid (NMDA) receptors trigger the generation of dendritic action potentials and subsequent nuclear Ca2+ waves that are instrumental in the control of gene expression. Although it is unclear how synaptic and nuclear Ca2+ are linked, activity-dependent increases in nuclear Ca2+ can activate transcription factors directly, such as DREAM, or indirectly, such as cAMP response element binding protein (CREB) via the regulatory kinase Ca2+/calmodulin-dependent kinase IV (CaMKIV) [1]. Hardingham and colleagues [7] have argued that synaptically evoked CREB activation requires only elevated nuclear Ca2+ and not the nuclear import of proteins, given that it occurred even when nuclear pores were blocked with wheat germ agglutinin. Although in this example nuclear Ca2+ dynamics might be sufficient, there is substantial evidence that synapse-to-nucleus communication also requires the nuclear import of cytosolic proteins.
Recent studies have shown that proteins enriched at neuronal synapses can accumulate in the nucleus in response to synaptic activity 8, 9, 10, 11. Indeed, many components of synaptic junctions exhibit a dual nuclear and synaptic localization 12, 13, 14 (Table 1), suggesting that they too shuttle between synaptic and nuclear compartments. Both importin-α and -β subunits of the nuclear import machinery have been identified at synaptic junctions by imaging and biochemical methods [15], which together with evidence that several components of the postsynaptic density (PSD) contain NLSs suggest that classical nuclear import mechanisms facilitate synapse-to-nucleus transport [13]. Therefore, regulated associations with importin subunits or synaptic anchors might determine the subcellular distribution of synaptic nuclear messengers. However, even as the number of proposed nuclear messengers continues to grow, fundamental questions regarding the mechanisms underlying direct synapse-to-nucleus transport remain unanswered. What has not yet emerged from these studies is how importin-associated, or non-classical nuclear signaling molecules, traverse the great lengths of neuronal dendrites to reach the nucleus. Here, we evaluate the evidence for postulated synapse to nucleus signaling messengers, explore the mechanisms involved and discuss how these molecules might reach the nucleus.
The nuclear translocation of components of junctional complexes is a well-documented mechanism for relaying extracellular stimuli to the nucleus [12]. Given the similarities between neuronal synapses and other cellular adhesion junctions, it is not surprising that synaptic junctions should also contain nucleocytoplasmic shuttling proteins (Table 1). Intercellular junctions are well suited to initiate nuclear signaling processes given their role in multicellular organization and proliferation. Cytoskeletal specializations that form at intercellular junctions such as PSDs and adherens plaques are enriched in cell adhesion molecules (CAMs), scaffolding proteins and signaling molecules including nuclear regulatory factors. CAMs in particular have been suggested to regulate nuclear signaling because they help to anchor various transcriptional coactivators in the cytosol 12, 16, 17, 18 (Figure 1). Biochemical in addition to large-scale proteomic profiling studies confirm that neuronal synapses contain a variety of CAMs, including cadherins, neurexins, integrins, neuroligins and syndecans, that might serve as nuclear signaling platforms 13, 19, 20, 21 (Table 1).
Synapses are also enriched in CAM-associated transcriptional regulators such as the catenin family members 13, 19, 20, 21, 22 (Table 1). These proteins can accumulate in the nucleus where they have been shown to regulate T-cell factor (TCF; also known as lymphoid enhancer-binding factor 1 [Lef-1]) and Kaiso-dependent transcription, among others [23]. Some examples include the widely studied β-catenin, which can act on TCF transcription factors and effect cMyc expression 24, 25, and catenin p120, which is thought to regulate transcription of rapysn [26] by sequestering the BTB/POZ repressor kaiso [27]. Neuronal synapses also contain junction γ-catenin (also known as plakoglobin), which like β-catenin regulates TCF-dependent transcription 28, 29. In addition to cadherin-based nuclear messengers, the PSD scaffolding molecule CASK, which binds to neurexin and syndecan 13, 19, can accumulate in the nucleus and regulate Tbr-1-dependent transcription [30] (Figure 1). Nuclear CASK regulates the expression of reelin [30]; a secreted extracellular matrix glycoprotein of which the gene is mutated in reeler mice, which possess impaired motor coordination [31]. The integrin-based c-Jun coactivator JAB-1 complex has also been identified in purified PSD fractions 19, 32. Indeed, c-Jun activity has been suggested to be essential for learning and memory given that mice lacking the N-terminal c-Jun kinase 2 (JNK2) exhibit impaired LTP [33]. The presence of these nuclear signaling molecules at synapses suggests that their nuclear transport participates in synaptic function.
Several additional synaptic scaffolding and/or cytoskeletal proteins exhibit dual nuclear and synaptic localization [13], including glucocorticoid receptor interacting protein 1 (GRIP1), zonula occluden (ZO)-1, ZO-2, synapse-associated protein 97 (SAP97), band 4.1, α-actinin-4 and ezrin. Indeed, specific roles in nuclear transport and transcriptional regulation have been proposed for several of these (Table 1). However, dual localization at synapses and nuclei might simply reflect different but stable subcellular distributions. One shared characteristic of most studies on nucleocytoplasmic shuttling is a paucity of data confirming the direct transport of proteins from synapses or other cellular junctions into the nucleus. Despite the large number of putative nuclear messengers, to our knowledge there is yet no demonstrative evidence of this phenomenon. In certain cases, increased nuclear accumulation has been correlated to decreases in plasma-membrane-associated proteins such as with ARVCF and ZO-1 34, 35. However, synaptic activity or junctional remodeling could regulate the synthesis or degradation of NLS-bearing nuclear factors resulting in transient changes in their subcellular distribution. Alternatively, differential splicing could generate variants with NLSs that could accumulate in the nucleus and be interpreted as nuclear translocation. This unusual mechanism results in the nuclear accumulation of NeK2 kinase [36]. Inhibition of protein synthesis or transcription during nuclear transport assays can, therefore, help allay some of these concerns. The lack of demonstrative data in this area also reflects the technological challenges underlying single-molecule imaging across long distances and the obscurity of the transport mechanisms involved, an issue we address later. However, considerably more is known about the mechanisms that trigger the nuclear import of proteins and their transport across the nuclear pore complex. In the following sections we address these mechanisms.
Section snippets
Nuclear import
Translocation across the nuclear membrane and into the nucleus is a highly regulated process [37]. In a majority of cases, proteins bound for the nucleus contain NLSs, which associate with importin-α subunits of the nuclear import machinery. In a landmark study, Thompson and colleagues [15] found that these importin subunits localize to distal dendritic sites, including synapses, in cultured Aplysia and rat hippocampal neurons. They found that NMDA receptor stimulation resulted in the nuclear
Specific NMDA-receptor-dependent messengers to the nucleus
Recent studies have identified synaptic proteins that shuttle into the nucleus in response to NMDA receptor activation 8, 9, 10, 11 (Figure 1). These studies are particularly interesting because NMDA receptor activation is pivotal for the control of plasticity-related gene expression and long-term memory formation. Like CAMs, NMDA receptors might function as nuclear signaling platforms given that they associate with large multiprotein complexes that include transcriptional regulators 51, 52.
Passive transport
A fundamental question in synapse-to-nucleus transport is how do proteins reach the nuclear pore from synapses that can be hundreds of microns away (Figure 3)? In addition to these spatial concerns, synapse-to-nucleus communication is challenged by temporal constraints: long-distance travel might result in the degradation of signaling molecules or the reversal of post-translational modifications required for nuclear import. Putative signal decay via dephosphorylation along dendrites or other
The current status of the synapse-to-nucleus signaling concept
The numerous examples of synaptically enriched components that accumulate in the nucleus reveal widespread nuclear control by synapses. However, there are important questions relating to the lack of demonstrative evidence that nuclear AIDA-1, Jacob, Abi-1 or CREB2 indeed have a synaptic origin. These questions are technically challenging to explore given the difficulty in tracking few, if not single, molecules across large portions of neurons. In the cell culture models usually used, it is
Acknowledgements
The work of B.J. is supported by the National Institutes of Health (www.nih.gov). M.R.K. is supported by the BMBF (www.bmbf.de), DFG (Deutsche Forschungsgemeinschaft; www.dfg.de), Land Saxony-Anhalt (http://www.med.uni-magdeburg.de/neuromd/index.html) and the Schram Foundation (http://www.stifterverband.de). We would like to thank Eckart D. Gundelfinger and U. Thomas for their critical comments on the manuscript and Anna Karpova and Marina Mikhaylova for help in preparing the figures.
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