Importin-mediated retrograde transport of CREB2 from distal processes to the nucleus in neurons

Abstract

Signals received at distal synapses of neurons must be conveyed to the nucleus to initiate the changes in transcription that underlie long-lasting synaptic plasticity. The presence of importin nuclear transporters and of select transcription factors at synapses raises the possibility that importins directly transport transcription factors from synapse to nucleus to modulate gene expression. Here, we show that cyclic AMP response element binding protein 2 (CREB2)/activating transcription factor 4 (ATF4), a transcriptional repressor that modulates long-term synaptic plasticity and memory, localizes to distal dendrites of rodent hippocampal neurons and neurites of Aplysia sensory neurons (SNs) and binds to specific importin ␣ isoforms. Binding of CREB2 to importin ␣ is required for its transport from distal dendrites to the soma and for its translocation into the nucleus. CREB2 accumulates in the nucleus during long-term depression (LTD) but not long-term potentiation of rodent hippocampal synapses, and during LTD but not long-term facilitation (LTF) of Aplysia sensory-motor synapses. Time-lapse microscopy of CREB2 tagged with a photoconvertible fluorescent protein further reveals retrograde transport of CREB2 from distal neurites to the nucleus of Aplysia SN during phenylalaninemethionine-arginine-phenylalanine-amide (FMRFamide)-induced LTD. Together, our findings indicate that CREB2 is a novel cargo of importin ␣ that translocates from distal synaptic sites to the nucleus after stimuli that induce LTD of neuronal synapses.Aplysia ͉ hippocampus ͉ transcription ͉ long-term depression ͉ dendra S ynaptic plasticity is critical to cognition and memory formation. Long-lasting forms of synaptic plasticity require both transcription and translation (1-3), and specific patterns and strengths of synaptic stimulation trigger changes in neuronal gene expression (4). However, relatively little is known about how signals are transported from synapse to nucleus to regulate transcription. Many types of synaptic stimulation induce depolarization and electrochemical signaling to rapidly alter transcription in the nucleus (5). A slower, more persistent mechanism of signaling to the nucleus involves the transport of soluble molecules from stimulated synapses to the nucleus (6, 7). Recent studies have indicated that this type of transport may involve the active nuclear import pathway (8-10).In the classical nuclear import pathway, proteins bearing nuclear localization signals (NLSs) bind an adaptor protein of the importin ␣ family, which in turn binds the importin ␤1 nuclear transporter. Importin ␤1 mediates facilitated translocation of this heterotrimeric complex across the nuclear pore. In neurons, importins may also function to carry signals from distal neuronal processes to the soma and into the nucleus (8-11). This finding raises a critical question: what are the synaptically localized cargoes of importins that translocate to the nucleus to alter transcription during synaptic plasticity?One attractive class of potential importin...

Distinct Molecular Regulation of Glycogen Synthase Kinase-3α Isozyme Controlled by Its N-terminal Region

“…This communication has been shown to be mediated by nuclear import of cytosolic proteins such as CREB, Jacob neuroprotein, NF-ATc, NFB, and Abelson interacting protein-1 (abi-1), which activate certain gene expression programs (65)(66)(67)(68)(69)(70). Proteases, in particular calpain, play important roles in these cellular activities (66,71). GSK-3␣ may thus link cytoplasmic calcium/calpain signaling with nuclear events.…”

“…In this respect, it is interesting to note that a rise in calcium influx has been implicated in synapse-to-nucleus communication regulating neuronal development, plasticity, and survival (63,64). This communication has been shown to be mediated by nuclear import of cytosolic proteins such as CREB, Jacob neuroprotein, NF-ATc, NFB, and Abelson interacting protein-1 (abi-1), which activate certain gene expression programs (65)(66)(67)(68)(69)(70). Proteases, in particular calpain, play important roles in these cellular activities (66,71).…”

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“…This binding is crucial since it allows Importins to translocate transcription factors, such as CREB2, into the nucleus and induce transcription-dependent forms of neuronal plasticity [22][23][24]. In addition to this synapseto-nucleus translocation during synaptic plasticity, Importins were shown to bring signals to the nucleus from injured axons [21].…”

Isolation of CA1 Nuclear Enriched Fractions from Hippocampal Slices to Study Activity-dependent Nuclear Import of Synapto-nuclear Messenger Proteins

“…Interestingly, some of these messengers only transit to the nucleus in response to specific synaptic stimuli. For example, retrograde transport of cyclic AMP response element binding protein 2 (CREB2) is induced by chemical LTD but not LTP 12 . Localized NMDAR-dependent synaptic stimulation drives CREB-regulated transcriptional coactivator (CRTC1) into the nucleus, a translocation process, which is involved in longterm hippocampal plasticity 4 .…”