Calcium signaling in synapse-to-nucleus communication

Abstract

Changes in the intracellular concentration of calcium ions in neurons are involved in neurite growth, development, and remodeling, regulation of neuronal excitability, increases and decreases in the strength of synaptic connections, and the activation of survival and programmed cell death pathways. An important aspect of the signals that trigger these processes is that they are frequently initiated in the form of glutamatergic neurotransmission within dendritic trees, while their completion involves specific changes in the patterns of genes expressed within neuronal nuclei. Accordingly, two prominent aims of research concerned with calcium signaling in neurons are determination of the mechanisms governing information conveyance between synapse and nucleus, and discovery of the rules dictating translation of specific patterns of inputs into appropriate and specific transcriptional responses. In this article, we present an overview of the avenues by which glutamatergic excitation of dendrites may be communicated to the neuronal nucleus and the primary calcium-dependent signaling pathways by which synaptic activity can invoke changes in neuronal gene expression programs.

Figure 1.

Sources of synaptic activity induced calcium signals. Here we consider five distinct routes by which Ca²⁺ may enter the neuronal cytoplasm. (1) Ca²⁺ may enter the cell from the extracellular space via ionotropic glutamate receptors, particularly NMDARs. (2) Ca²⁺ may also pass from the extracellular space into the cytoplasm by way of VDCCs, most notably the dihydropine-sensitive class of high voltage-activated, or L-type, VDCCs. (3) Stimulation of mGluRs can trigger release of Ca²⁺ into the cytoplasm from intracellular Ca²⁺ stores like ER: synaptically released glutamate activates mGluRs, which are coupled via G_q/11 GTP-binding proteins to PLC. Activated PLC cleaves membrane-bound PIP₂ to yield DAG and soluble IP₃, which may then diffuse to and activate IP₃Rs on the ER membrane. (4) Cytosolic Ca²⁺ signals originating from any of the ligand-gated glutamate receptors, VDCCs, or from IP₃Rs can be amplified via the Ca²⁺-dependent activation of RyRs and subsequent internal release of Ca²⁺ from intracellular stores. (5) Synaptically released glutamate may activate mGluRs on the inner nuclear envelope subsequent to being taken up by EAATs on the plasma membrane first into the cytosol, and then by EAATs on the nuclear envelope into the nuclear lumen. Stimulation of intranuclear mGluRs may consequently lead to the release of Ca²⁺ directly into the nucleus from IP₃Rs localized on the inner nuclear envelope.

Figure 2.

Activity-dependent gene expression in neurons is regulated by distinct calcium signaling pathways. Distinct activity-dependent genomic responses may be explained by the differential activation of regulatory signaling cascades and transcription factors. (1) Ca²⁺ influx through L-type VDCCs and synaptically activated NMDARs binds to and activates CaM, leading to activation of the GTP-binding protein Ras and subsequent induction of the Ras/MAPK signaling pathway. Ras binds and activates the mitogen activated protein kinase kinase kinase Raf. Raf in turn phosphoyrlates and activates MEK, which phosphoyrlates and activates ERK 1 and 2. Following its translocation into the nucleus, ERK can phosphorylate intermediate kinases like RSK2, which in turn phosphorylate CREB. ERK may also phosphorylate and activate the TCF-family transcription factors, which upon forming a complex with SRF may initiate gene transcription downstream from SRF binding sites. (2) CaM activation by Ca²⁺ entering through NMDARs and VDCCs, but especially through L-type VDCCs, also stimulates the CaMK signal transduction cascade. Ca²⁺/CaM binds to and activates CaMKK. Next, CaMKK phosphorylates and activates cytoplasmic CaMKI, which can in turn translocate into the nucleus (Sakagami et al. 2005; Wayman et al. 2008). CaMKK also phosphorylates Ca²⁺/CaM-bound CaMKIV. CaMKIV and perhaps also CaMKI can phosphorylate the transcription factor CREB (Wayman et al. 2008). Stimulation of CREB-mediated transcription, however, requires the additional activation by CaMKIV of the CREB coactivator CBP. It follows that VDCC and NMDAR-mediated [Ca²⁺]_i increases and subsequent activation of Ras/MAPK pathways are sufficient to induce TCF/SRF-dependent transcription. However, activation of CREB-dependent transcription necessitates the additional Ca²⁺-dependent stimulation of CaMKIV and CBP by nuclear Ca²⁺.

Figure 3.

Possible routes of communication between synapse and nucleus. Schematic illustration of four pathways by which information impinging onto synapses may be conveyed to the nucleus. (A) Glutamatergic activation of synaptic AMPARs and NMDARs trigger increases in Ca²⁺ within synaptic spines. These Ca²⁺ increases may be amplified locally by Ca²⁺-induced Ca²⁺ release via RyRs on spinous ER. [Ca²⁺]_i increases within spines trigger the activation of numerous NLS-harboring second messengers (cargo-NLS) and the mobilization of importin. Importin/cargo complexes then diffuse or are actively transported through the dendritic cytosol to invade the soma and nucleus. (B) Synaptic activity evokes synaptic EPSPs, which travel along the plasma membrane to the soma. When the summation of inputs exceeds firing threshold, an action potential is triggered. Membrane depolarizations accompanying EPSPs and action potentials activate somatic and perisomatic L-type VDCCs, which allow Ca²⁺ to flow down its concentration gradient from the extracellular space into the soma, from where it crosses the nuclear envelope to invade the nucleus. (C) Robust stimulation of glutamatergic afferents can result in the activation of perisynaptic G_q/11-protein-coupled glutamate receptors, or mGluRs, which in turn trigger a signaling cascade that results in the production of the second messenger IP₃. IP₃ subsequently binds to and activates IP₃Rs on the ER membrane, causing their Ca²⁺-permeable channels to open and allowing Ca²⁺ to flow down its concentration gradient from the ER lumen into the cytosol. Ca²⁺, an IP₃R coagonist, may then stimulate neighboring IP₃Rs, thus triggering a propagating wave of regenerative internal Ca²⁺ release. Robust Ca²⁺ waves that invade the soma are able to freely cross the nuclear envelope and generate robust nuclear Ca²⁺ transients. (D) Stimulation of ligand-operated and voltage-dependent ion channels in the plasma membrane of spines evokes local currents across the plasma membrane, I_pm. Either as a result of capacitative conduction of plasma membrane potential fluctuations combined with Ca²⁺ uptake into the ER via sarcoendoplasmic reticulum Ca²⁺ ATPases, or as a consequence of the release of Ca²⁺ via RyRs and IP₃Rs localized to spines and dendrites, synaptic currents are predicted to evoke an electrotonic signal across and along the ER membrane, such that synaptic currents flow simultaneously through the cytosol (I_i) and within the ER (I_ER) toward the soma. Summation of many such excitatory electrotonic signals can give rise to an EPSP-like depolarization across the nuclear envelope. Importantly, action potentials initiating at the soma would also result in propagating electrotonic signals along the ER. In this case, however, the resultant changes in potential across the nuclear envelope would be hyperpolarizing, thereby distinguishing these signals from others that originate distally in synaptic spines.

Figure 4.

Extrasynaptic NMDARs antagonize synaptic NMDAR-dependent synapse-to-nucleus signaling. NMDAR-dependent Ca²⁺ signaling promotes neuronal survival, but can also cause cell degeneration and apoptotic death. The location of the activated NMDAR pool specifies the transcriptional response. In particular, Ca²⁺ influx through synaptic NMDARs leads to the activation and nuclear translocation of ERK followed by phosphorylation of the neuroprotection-associated transcription factor, CREB, and transcription of prosurvival gene products including the prosurvival neurotrophic factor BDNF. Synaptic NMDAR activity also induces the nuclear export of HDACs and of the proapoptotic transcription factor FOXO. Export of HDACs allows for the acetylation and disinhibition of transcription mediated by prosurvival transcription factors such as MEF2 and CREB. Conversely, ES-NMDAR-mediated Ca²⁺ entry triggers transcription of the proapoptotic Ca²⁺-activated chloride channel Clca1, triggers the rapid dephosphorylation and inactivation of ERK, thus preventing its nuclear translocation, interferes with the nuclear export of HDACs, and triggers the rapid nuclear import of FOXO proteins. Thus, whereas activation of synaptic NMDARs promotes the transcription of neuroprotective target genes via CREB and MEF2, ES-NMDAR activation actively represses CREB- and MEF-dependent gene transcription, and leads to the transcription of genes associated with neuronal degeneration and apoptotic cell death.

Figure 5.

Epigenetic mechanisms in synaptic activity and Ca²⁺-dependent transcriptional regulation. Synaptic activity-dependent [Ca²⁺]_i increases may influence chromatin structure—and therewith access of transcription factors and regulatory enzymes to their specific interaction domains—through influences on histone acetylation and DNA methylation. Synaptic activity-dependent Ca²⁺ signals acting through CaM and CAMK and Ras/MAPK signaling cascades can, for instance, activate HATs such as CBP and p300. These transcriptional coactivators catalyze the posttranslational acetylation of histones, leading to chromatin relaxation, increased access of transcription factors to their respective DNA binding domains, and up-regulation of gene transcription in affected genomic regions. Alternately, nuclear Ca²⁺ increases associated with synaptic activity may trigger the CAMK-dependent phosphorylation and dissociation of methyl CpG- binding proteins like MeCP2 from methylated sequences of DNA as well as the nuclear export of the HDACs with which they interact. HDAC activity results in the deacetylation of histones and the subsequent compression of chromatin, leading to a suppression of transcriptional activity. MeCP2 dissociation and HDAC export thus induce a derepression of gene transcription. Ca²⁺ influx associated with synaptic activity is linked also to de novo DNA methylation via DNMTs such as DNMT3a, and to active DNA demethylation via dMTases such as GADD45a. Note that, although not shown here, histone phosphorylation, ubiquitination, and methylation also contribute to the epigenetic regulation of gene expression (Levenson and Sweatt 2006), and may likewise be involved in Ca²⁺-dependent synapse-to-nucleus communication.