Densin-180 Controls the Trafficking and Signaling of L-Type Voltage-Gated Cav1.2 Ca2+ Channels at Excitatory Synapses

Rem2 interacts with CaMKII at synapses and restricts long-term potentiation in hippocampus

Synaptic plasticity, the process whereby neuronal connections are either strengthened or weakened in response to stereotyped forms of stimulation, is widely believed to represent the molecular mechanism that underlies learning and memory. The holoenzyme calcium/calmodulin-dependent protein kinase II (CaMKII) plays a well-established and critical role in the induction of a variety of forms of synaptic plasticity such as long-term potentiation (LTP), long-term depression (LTD) and depotentiation. Previously, we identified the GTPase Rem2 as a potent, endogenous inhibitor of CaMKII. Here, we report that knock out of Rem2 enhances LTP at the Schaffer collateral to CA1 synapse in hippocampus, consistent with an inhibitory action of Rem2 on CaMKII in vivo. Further, re-expression of WT Rem2 rescues the enhanced LTP observed in slices obtained from Rem2 conditional knock out (cKO) mice, while expression of a mutant Rem2 construct that is unable to inhibit CaMKII in vitro fails to rescue increased LTP. In addition, we demonstrate that CaMKII and Rem2 interact in dendritic spines using a 2pFLIM-FRET approach. Taken together, our data lead us to propose that Rem2 serves as a brake on synaptic potentiation via inhibition of CaMKII activity. Further, the enhanced LTP phenotype we observe in Rem2 cKO slices reveals a previously unknown role for Rem2 in the negative regulation of CaMKII function.

Rem2 interacts with CaMKII at synapses and restricts long-term potentiation in hippocampus

Synaptic plasticity, the process whereby neuronal connections are either strengthened or weakened in response to stereotyped forms of stimulation, is widely believed to represent the molecular mechanism that underlies learning and memory. The holoenzyme CaMKII plays a well-established and critical role in the induction of a variety of forms of synaptic plasticity such as long-term potentiation (LTP), long-term depression (LTD) and depotentiation. Previously, we identified the GTPase Rem2 as a potent, endogenous inhibitor of CaMKII. Here, we report that knock out of Rem2 enhances LTP at the Schaffer collateral to CA1 synapse in hippocampus, consistent with an inhibitory action of Rem2 on CaMKII in vivo. Further, re-expression of WT Rem2 rescues the enhanced LTP observed in slices obtained from Rem2 conditional knock out (cKO) mice, while expression of a mutant Rem2 construct that is unable to inhibit CaMKII in vitro fails to rescue increased LTP. In addition, we demonstrate that CaMKII and Rem2 interact in dendritic spines using a 2pFLIM-FRET approach. Taken together, our data lead us to propose that Rem2 serves as a brake on runaway synaptic potentiation via inhibition of CaMKII activity. Further, the enhanced LTP phenotype we observe in Rem2 cKO slices reveals a previously unknown role for Rem2 in the negative regulation of CaMKII function.

CaV 2.1 α1 subunit motifs that control presynaptic CaV 2.1 subtype abundance are distinct from CaV 2.1 preference

Presynaptic voltage-gated Ca2+ channel (CaV ) subtype abundance at mammalian synapses regulates synaptic transmission in health and disease. In the mammalian central nervous system (CNS), most presynaptic terminals are CaV 2.1 dominant with a developmental reduction in CaV 2.2 and CaV 2.3 levels, and CaV 2 subtype levels are altered in various diseases. However, the molecular mechanisms controlling presynaptic CaV 2 subtype levels are largely unsolved. Because the CaV 2 α1  subunit cytoplasmic regions contain varying levels of sequence conservation, these regions are proposed to control presynaptic CaV 2 subtype preference and abundance. To investigate the potential role of these regions, we expressed chimeric CaV 2.1 α1  subunits containing swapped motifs with the CaV 2.2 and CaV 2.3 α1  subunit on a CaV 2.1/CaV 2.2 null background at the calyx of Held presynaptic terminals. We found that expression of CaV 2.1 α1  subunit chimeras containing the CaV 2.3 loop II-III region or cytoplasmic C-terminus (CT) resulted in a large reduction of presynaptic Ca2+ currents compared to the CaV 2.1 α1  subunit. However, the Ca2+ current sensitivity to the CaV 2.1 blocker agatoxin-IVA was the same between the chimeras and the CaV 2.1 α1  subunit. Additionally, we found no reduction in presynaptic Ca2+ currents with CaV 2.1/2.2 cytoplasmic CT chimeras. We conclude that the motifs in the CaV 2.1 loop II-III and CT do not individually regulate CaV 2.1 preference, although these motifs control CaV 2.1 levels and the CaV 2.3 CT contains motifs that negatively regulate presynaptic CaV 2.3 levels. We propose that the motifs controlling presynaptic CaV 2.1 preference are distinct from those regulating CaV 2.1 levels and may act synergistically to impact pathways regulating CaV 2.1 preference and abundance. KEY POINTS: Presynaptic CaV 2 subtype abundance regulates neuronal circuit properties, although the mechanisms regulating presynaptic CaV 2 subtype abundance and preference remain enigmatic. The CaV α1  subunit determines subtype and contains multiple motifs implicated in regulating presynaptic subtype abundance and preference. The CaV 2.1 α1  subunit domain II-III loop and cytoplasmic C-terminus are positive regulators of presynaptic CaV 2.1 abundance but do not regulate preference. The CaV 2.3 α1  subunit cytoplasmic C-terminus negatively regulates presynaptic CaV 2 subtype abundance but not preference, whereas the CaV 2.2 α1  subunit cytoplasmic C-terminus is not a key regulator of presynaptic CaV 2 subtype abundance or preference. The CaV 2 α1  subunit motifs determining the presynaptic CaV 2 preference are distinct from abundance.

Aberrant DJ-1 expression underlies L-type calcium channel hypoactivity in dendrites in tuberous sclerosis complex and Alzheimer's disease

L-type voltage-gated calcium (Ca2+) channels (L-VGCC) dysfunction is implicated in several neurological and psychiatric diseases. While a popular therapeutic target, it is unknown whether molecular mechanisms leading to disrupted L-VGCC across neurodegenerative disorders are conserved. Importantly, L-VGCC integrate synaptic signals to facilitate a plethora of cellular mechanisms; however, mechanisms that regulate L-VGCC channel density and subcellular compartmentalization are understudied. Herein, we report that in disease models with overactive mammalian target of rapamycin complex 1 (mTORC1) signaling (or mTORopathies), deficits in dendritic L-VGCC activity are associated with increased expression of the RNA-binding protein (RBP) Parkinsonism-associated deglycase (DJ-1). DJ-1 binds the mRNA coding for the alpha and auxiliary Ca2+ channel subunits CaV1.2 and α2δ2, and represses their mRNA translation, only in the disease states, specifically preclinical models of tuberous sclerosis complex (TSC) and Alzheimer's disease (AD). In agreement, DJ-1-mediated repression of CaV1.2/α2δ2 protein synthesis in dendrites is exaggerated in mouse models of AD and TSC, resulting in deficits in dendritic L-VGCC calcium activity. Finding of DJ-1-regulated L-VGCC activity in dendrites in TSC and AD provides a unique signaling pathway that can be targeted in clinical mTORopathies.

Excitation-transcription coupling, neuronal gene expression and synaptic plasticity

Excitation-transcription coupling (E-TC) links synaptic and cellular activity to nuclear gene transcription. It is generally accepted that E-TC makes a crucial contribution to learning and memory through its role in underpinning long-lasting synaptic enhancement in late-phase long-term potentiation and has more recently been linked to late-phase long-term depression: both processes require de novo gene transcription, mRNA translation and protein synthesis. E-TC begins with the activation of glutamate-gated N-methyl-D-aspartate-type receptors and voltage-gated L-type Ca2+ channels at the membrane and culminates in the activation of transcription factors in the nucleus. These receptors and ion channels mediate E-TC through mechanisms that include long-range signalling from the synapse to the nucleus and local interactions within dendritic spines, among other possibilities. Growing experimental evidence links these E-TC mechanisms to late-phase long-term potentiation and learning and memory. These advances in our understanding of the molecular mechanisms of E-TC mean that future efforts can focus on understanding its mesoscale functions and how it regulates neuronal network activity and behaviour in physiological and pathological conditions.

Regionally restricted modulation of Sam68 expression and Arhgef9 alternative splicing in the hippocampus of a murine model of multiple sclerosis

Multiple sclerosis (MS) and its preclinical models are characterized by marked changes in neuroplasticity, including excitatory/inhibitory imbalance and synaptic dysfunction that are believed to underlie the progressive cognitive impairment (CI), which represents a significant clinical hallmark of the disease. In this study, we investigated several parameters of neuroplasticity in the hippocampus of the experimental autoimmune encephalomyelitis (EAE) SJL/J mouse model, characterized by rostral inflammatory and demyelinating lesions similar to Relapsing-Remitting MS. By combining morphological and molecular analyses, we found that the hippocampus undergoes extensive inflammation in EAE-mice, more pronounced in the CA3 and dentate gyrus (DG) subfields than in the CA1, associated with changes in GABAergic circuitry, as indicated by the increased expression of the interneuron marker Parvalbumin selectively in CA3. By laser-microdissection, we investigated the impact of EAE on the alternative splicing of Arhgef9, a gene encoding a post-synaptic protein playing an essential role in GABAergic synapses and whose mutations have been related to CI and epilepsy. Our results indicate that EAE induces a specific increase in inclusion of the alternative exon 11a only in the CA3 and DG subfields, in line with the higher local levels of inflammation. Consistently, we found a region-specific downregulation of Sam68, a splicing-factor that represses this splicing event. Collectively, our findings confirm a regionalized distribution of inflammation in the hippocampus of EAE-mice. Moreover, since neuronal circuit rearrangement and dynamic remodeling of structural components of the synapse are key processes that contribute to neuroplasticity, our study suggests potential new molecular players involved in EAE-induced hippocampal dysfunction.

Bipolar‐associated miR ‐499‐5p controls neuroplasticity by downregulating the Cav1.2 subunit CACNB2

Bipolar disorder (BD) is a chronic mood disorder characterized by manic and depressive episodes. Dysregulation of neuroplasticity and calcium homeostasis are frequently observed in BD patients, but the underlying molecular mechanisms are largely unknown. Here, we show that miR‐499‐5p regulates dendritogenesis and cognitive function by downregulating the BD risk gene CACNB2. miR‐499‐5p expression is increased in peripheral blood of BD patients, as well as in the hippocampus of rats which underwent juvenile social isolation. In rat hippocampal neurons, miR‐499‐5p impairs dendritogenesis and reduces surface expression and activity of the L‐type calcium channel Cav1.2. We further identified CACNB2, which encodes a regulatory β‐subunit of Cav1.2, as a direct functional target of miR‐499‐5p in neurons. miR‐499‐5p overexpression in the hippocampus in vivo induces short‐term memory impairments selectively in rats haploinsufficient for the Cav1.2 pore forming subunit Cacna1c. In humans, miR‐499‐5p expression is negatively associated with gray matter volumes of the left superior temporal gyrus, a region implicated in auditory and emotional processing. We propose that stress‐induced miR‐499‐5p overexpression contributes to dendritic impairments, deregulated calcium homeostasis, and neurocognitive dysfunction in BD.

The voltage-gated Ca2+ channel subunit α2δ-4 regulates locomotor behavior and sensorimotor gating in mice

Voltage-gated Ca2+ channels are critical for the development and mature function of the nervous system. Variants in the CACNA2D4 gene encoding the α2δ-4 auxiliary subunit of these channels are associated with neuropsychiatric and neurodevelopmental disorders. α2δ-4 is prominently expressed in the retina and is crucial for vision, but extra-retinal functions of α2δ-4 have not been investigated. Here, we sought to fill this gap by analyzing the behavioral phenotypes of α2δ-4 knockout (KO) mice. α2δ-4 KO mice (both males and females) exhibited significant impairments in prepulse inhibition that were unlikely to result from the modestly elevated auditory brainstem response thresholds. Whereas α2δ-4 KO mice of both sexes were hyperactive in various assays, only females showed impaired motor coordination in the rotarod assay. α2δ-4 KO mice exhibited anxiolytic and anti-depressive behaviors in the elevated plus maze and tail suspension tests, respectively. Our results reveal an unexpected role for α2δ-4 in sensorimotor gating and motor function and identify α2δ-4 KO mice as a novel model for studying the pathophysiology associated with CACNA2D4 variants.

Mechanisms and Physiological Implications of Cooperative Gating of Ion Channels Clusters

Ion channels play a central role in the regulation of nearly every cellular process. Dating back to the classic 1952 Hodgkin-Huxley model of the generation of the action potential, ion channels have always been thought of as independent agents. A myriad of recent experimental findings exploiting advances in electrophysiology, structural biology, and imaging techniques, however, have posed a serious challenge to this long-held axiom as several classes of ion channels appear to open and close in a coordinated, cooperative manner. Ion channel cooperativity ranges from variable-sized oligomeric cooperative gating in voltage-gated, dihydropyridine-sensitive Ca_v1.2 and Ca_v1.3 channels to obligatory dimeric assembly and gating of voltage-gated Na_v1.5 channels. Potassium channels, transient receptor potential channels, hyperpolarization cyclic nucleotide-activated channels, ryanodine receptors (RyRs), and inositol trisphosphate receptors (IP₃Rs) have also been shown to gate cooperatively. The implications of cooperative gating of these ion channels range from fine tuning excitation-contraction coupling in muscle cells to regulating cardiac function and vascular tone, to modulation of action potential and conduction velocity in neurons and cardiac cells, and to control of pace-making activity in the heart. In this review, we discuss the mechanisms leading to cooperative gating of ion channels, their physiological consequences and how alterations in cooperative gating of ion channels may induce a range of clinically significant pathologies.

Regulation of neuronal excitation-transcription coupling by Kv2.1-induced clustering of somatic L-type Ca2+ channels at ER-PM junctions

In hippocampal neurons, gene expression is triggered by electrical activity and Ca²⁺ entry via L-type Cav1.2 channels in a process called excitation–transcription coupling. We identified a domain on the voltage-gated K⁺ channel Kv2.1 that promotes the clustering of L-type Cav1.2 channels at endoplasmic reticulum–plasma membrane junctions in the soma of neurons. Importantly, we discovered by disrupting this domain that the Kv2.1-mediated clustering of Cav1.2 at this somatic microdomain is critical for depolarization-induced excitation–transcription coupling.

In mammalian brain neurons, membrane depolarization leads to voltage-gated Ca²⁺ channel-mediated Ca²⁺ influx that triggers diverse cellular responses, including gene expression, in a process termed excitation–transcription coupling. Neuronal L-type Ca²⁺ channels, which have prominent populations on the soma and distal dendrites of hippocampal neurons, play a privileged role in excitation–transcription coupling. The voltage-gated K⁺ channel Kv2.1 organizes signaling complexes containing the L-type Ca²⁺ channel Cav1.2 at somatic endoplasmic reticulum–plasma membrane junctions. This leads to enhanced clustering of Cav1.2 channels, increasing their activity. However, the downstream consequences of the Kv2.1-mediated regulation of Cav1.2 localization and function on excitation–transcription coupling are not known. Here, we have identified a region between residues 478 to 486 of Kv2.1’s C terminus that mediates the Kv2.1-dependent clustering of Cav1.2. By disrupting this Ca²⁺ channel association domain with either mutations or with a cell-penetrating interfering peptide, we blocked the Kv2.1-mediated clustering of Cav1.2 at endoplasmic reticulum–plasma membrane junctions and the subsequent enhancement of its channel activity and somatic Ca²⁺ signals without affecting the clustering of Kv2.1. These interventions abolished the depolarization-induced and L-type Ca²⁺ channel-dependent phosphorylation of the transcription factor CREB and the subsequent expression of c-Fos in hippocampal neurons. Our findings support a model whereby the Kv2.1-Ca²⁺ channel association domain-mediated clustering of Cav1.2 channels imparts a mechanism to control somatic Ca²⁺ signals that couple neuronal excitation to gene expression.

Development of phenotypic assays for identifying novel blockers of L-type calcium channels in neurons

L-type calcium channels (LTCCs) are highly expressed in the heart and brain and are critical for cardiac and neuronal functions. LTCC-blocking drugs have a long and successful record in the clinic for treating cardiovascular disorders. In contrast, establishment of their efficacy for indications of the central nervous system remains challenging given the tendency of existing LTCC drugs being functionally and mechanistically more selective for peripheral tissues. LTCCs in vivo are large macromolecular complexes consisting of a pore-forming subunit and other modulatory proteins, some of which may be neuro-specific and potentially harbor mechanisms for neuronal selectivity. To exploit the possibility of identifying mechanistically novel and/or neuro-selective blockers, we developed two phenotypic assays—a calcium flux-based primary screening assay and a patch clamp secondary assay, using rat primary cortical cultures. We screened a library comprised of 1278 known bioactive agents and successfully identified a majority of the potent LTCC-blocking drugs in the library. Significantly, we identified a previously unrecognized LTCC blocker with a novel mechanism, which was corroborated by patch clamp and binding studies. As such, these phenotypic assays are robust and represent an important step towards identifying mechanistically novel and neuro-selective LTCC blockers.

Bilateral histone deacetylase 1 and 2 activity and enrichment at unique genes following induction of long-term potentiation in vivo

Long-term potentiation (LTP) is a synaptic plasticity mechanism critical to long-term memory. LTP induced in vivo is characterized by altered transcriptional activity, including a period of upregulation of gene expression which is followed by a later dominant downregulation. This temporal shift to downregulated gene expression is predicted to be partly mediated by epigenetic inhibitors of gene expression, such as histone deacetylases (HDACs). Further, pharmacological inhibitors of HDAC activity have previously been shown to enhance LTP persistence in vitro. To explore the contribution of HDACs to the persistence of LTP in vivo, we examined HDAC1 and HDAC2 activity over a 24 hr period following unilateral LTP induction in the dentate gyrus of freely moving rats. Surprisingly, we found significant changes in HDAC1 and HDAC2 activity in both the stimulated as well as the unstimulated hemispheres, with the largest increase in activity occurring bilaterally, 20 min after LTP stimulation. During this time point of heightened activity, chromatin immunoprecipitation assays showed that both HDAC1 and HDAC2 were enriched at distinct sets of genes within each hemispheres. Further, the HDAC inhibitor Trichostatin A enhanced an intermediate phase of LTP lasting days, which has not previously been associated with altered transcription. The inhibitor had no effect on the persistence of LTP lasting weeks. Together, these data suggest that HDAC activity early after the induction of LTP may negatively regulate plasticity-related gene expression that is involved in the initial stabilization of LTP, but not its long-term maintenance.

Role of densin-180 in mouse ventral hippocampal neurons in 24-hr retention of contextual fear conditioning

INTRODUCTION

Densin-180 interacts with postsynaptic molecules including calcium/calmodulin-dependent protein kinase IIα (CaMKIIα) but its function in learning and memory process has been unclear.

METHODS

To investigate a role of hippocampal densin-180 in contextual fear conditioning (CFC) learning and memory processes, knockdown (KD) of densin-180 in hippocampal subareas was applied.

RESULTS

First, ventral hippocampal (vHC) densin-180 KD impaired single-trial CFC (stCFC) memory one day later. stCFC caused freezing behaviors to reach the peak about one hour later in both control and KD mice, but then freezing was disappeared at 2 hr postshock in KD mice. Second, stCFC caused an immediate and transient reduction of vHC densin-180 in control mice, which was not observed in KD mice. Third, stCFC caused phosphorylated-T286 (p-T286) CaMKIIα to change similarly to densin-180, but p-T305 CaMKIIα was increased 1 hr later in control mice. In KD mice, these effects were gone. Moreover, both basal levels of p-T286 and p-T305 CaMKIIα were reduced without change in total CaMKIIα in KD mice. Fourth, we found double-trial CFC (dtCFC) memory acquisition and retrieval kinetics were different from those of stCFC in vHC KD mice. In addition, densin-180 in dorsal hippocampal area appeared to play its unique role during the very early retrieval period of both CFC memories.

CONCLUSION

This study shows that vHC densin-180 is necessary for stCFC memory formation and retrieval and suggests that both densin-180 and p-T305 CaMKIIα at 1 ~ 2 hr postshock are important for stCFC memory formation. We conclude that roles of hippocampal neuronal densin-180 in CFC are temporally dynamic and differential depending on the pattern of conditioning stimuli and its location along the dorsoventral axis of hippocampal formation.

Tetraspanin-7 regulation of L-type voltage-dependent calcium channels controls pancreatic β-cell insulin secretion

KEY POINTS

Tetraspanin (TSPAN) proteins regulate many biological processes, including intracellular calcium (Ca2+ ) handling. TSPAN-7 is enriched in pancreatic islet cells; however, the function of islet TSPAN-7 has not been identified. Here, we characterize how β-cell TSPAN-7 regulates Ca2+ handling and hormone secretion. We find that TSPAN-7 reduces β-cell glucose-stimulated Ca2+ entry, slows Ca2+ oscillation frequency and decreases glucose-stimulated insulin secretion. TSPAN-7 controls β-cell function through a direct interaction with L-type voltage-dependent Ca2+ channels (CaV 1.2 and CaV 1.3), which reduces channel Ca2+ conductance. TSPAN-7 slows activation of CaV 1.2 and accelerates recovery from voltage-dependent inactivation; TSPAN-7 also slows CaV 1.3 inactivation kinetics. These findings strongly implicate TSPAN-7 as a key regulator in determining the set-point of glucose-stimulated Ca2+ influx and insulin secretion.

ABSTRACT

Glucose-stimulated insulin secretion (GSIS) is regulated by calcium (Ca2+ ) entry into pancreatic β-cells through voltage-dependent Ca2+ (CaV ) channels. Tetraspanin (TSPAN) transmembrane proteins control Ca2+ handling, and thus they may also modulate GSIS. TSPAN-7 is the most abundant islet TSPAN and immunostaining of mouse and human pancreatic slices shows that TSPAN-7 is highly expressed in β- and α-cells; however, the function of islet TSPAN-7 has not been determined. Here, we show that TSPAN-7 knockdown (KD) increases glucose-stimulated Ca2+ influx into mouse and human β-cells. Additionally, mouse β-cell Ca2+ oscillation frequency was accelerated by TSPAN-7 KD. Because TSPAN-7 KD also enhanced Ca2+ entry when membrane potential was clamped with depolarization, the effect of TSPAN-7 on CaV channel activity was examined. TSPAN-7 KD enhanced L-type CaV currents in mouse and human β-cells. Conversely, heterologous expression of TSPAN-7 with CaV 1.2 and CaV 1.3 L-type CaV channels decreased CaV currents and reduced Ca2+ influx through both channels. This was presumably the result of a direct interaction of TSPAN-7 and L-type CaV channels because TSPAN-7 coimmunoprecipitated with both CaV 1.2 and CaV 1.3 from primary human β-cells and from a heterologous expression system. Finally, TSPAN-7 KD in human β-cells increased basal (5.6 mM glucose) and stimulated (45 mM KCl + 14 mM glucose) insulin secretion. These findings strongly suggest that TSPAN-7 modulation of β-cell L-type CaV channels is a key determinant of β-cell glucose-stimulated Ca2+ entry and thus the set-point of GSIS.

Molecular architecture of postsynaptic Interactomes

The postsynaptic density (PSD) plays an essential role in the organization of the synaptic signaling machinery. It contains a set of core scaffolding proteins that provide the backbone to PSD protein-protein interaction networks (PINs). These core scaffolding proteins can be seen as three principal layers classified by protein family, with DLG proteins being at the top, SHANKs along the bottom, and DLGAPs connecting the two layers. Early studies utilizing yeast two hybrid enabled the identification of direct protein-protein interactions (PPIs) within the multiple layers of scaffolding proteins. More recently, mass-spectrometry has allowed the characterization of whole interactomes within the PSD. This expansion of knowledge has further solidified the centrality of core scaffolding family members within synaptic PINs and provided context for their role in neuronal development and synaptic function. Here, we discuss the scaffolding machinery of the PSD, their essential functions in the organization of synaptic PINs, along with their relationship to neuronal processes found to be impaired in complex brain disorders.

Interactions with PDZ proteins diversify voltage-gated calcium channel signaling

Voltage-gated Ca²⁺ (Ca_V ) channels are crucial for neuronal excitability and synaptic transmission upon depolarization. Their properties in vivo are modulated by their interaction with a variety of scaffolding proteins. Such interactions can influence the function and localization of Ca_V channels, as well as their coupling to intracellular second messengers and regulatory pathways, thus amplifying their signaling potential. Among these scaffolding proteins, a subset of PDZ (postsynaptic density-95, Drosophila discs-large, and zona occludens)-domain containing proteins play diverse roles in modulating Ca_V channel properties. At the presynaptic terminal, PDZ proteins enrich Ca_V channels in the active zone, enabling neurotransmitter release by maintaining a tight and vital link between channels and vesicles. In the postsynaptic density, these interactions are essential in regulating dendritic spine morphology and postsynaptic signaling cascades. In this review, we highlight the studies that demonstrate dynamic regulations of neuronal Ca_V channels by PDZ proteins. We discuss the role of PDZ proteins in controlling channel activity, regulating channel cell surface density, and influencing channel-mediated downstream signaling events. We highlight the importance of PDZ protein regulations of Ca_V channels and evaluate the link between this regulatory effect and human disease.

Regulation of cardiovascular calcium channel activity by post-translational modifications or interacting proteins

Voltage-gated calcium channels are the major pathway for Ca²⁺ influx to initiate the contraction of smooth and cardiac muscles. Alterations of calcium channel function have been implicated in multiple cardiovascular diseases, such as hypertension, atrial fibrillation, and long QT syndrome. Post-translational modifications do expand cardiovascular calcium channel structure and function to affect processes such as channel trafficking or polyubiquitination by two E3 ubiquitin ligases, Ret finger protein 2 (Rfp2) or murine double minute 2 protein (Mdm2). Additionally, biophysical property such as Ca²⁺-dependent inactivation (CDI) could be altered through binding of calmodulin, or channel activity could be modulated via S-nitrosylation by nitric oxide and phosphorylation by protein kinases or by interacting protein partners, such as galectin-1 and Rem. Understanding how cardiovascular calcium channel function is post-translationally remodeled under distinctive disease conditions will provide better information about calcium channel-related disease mechanisms and improve the development of more selective therapeutic agents for cardiovascular diseases.

Functional Annotation of the Transcriptome of the Pig, Sus scrofa, Based Upon Network Analysis of an RNAseq Transcriptional Atlas

The domestic pig (Sus scrofa) is both an economically important livestock species and a model for biomedical research. Two highly contiguous pig reference genomes have recently been released. To support functional annotation of the pig genomes and comparative analysis with large human transcriptomic data sets, we aimed to create a pig gene expression atlas. To achieve this objective, we extended a previous approach developed for the chicken. We downloaded RNAseq data sets from public repositories, down-sampled to a common depth, and quantified expression against a reference transcriptome using the mRNA quantitation tool, Kallisto. We then used the network analysis tool Graphia to identify clusters of transcripts that were coexpressed across the merged data set. Consistent with the principle of guilt-by-association, we identified coexpression clusters that were highly tissue or cell-type restricted and contained transcription factors that have previously been implicated in lineage determination. Other clusters were enriched for transcripts associated with biological processes, such as the cell cycle and oxidative phosphorylation. The same approach was used to identify coexpression clusters within RNAseq data from multiple individual liver and brain samples, highlighting cell type, process, and region-specific gene expression. Evidence of conserved expression can add confidence to assignment of orthology between pig and human genes. Many transcripts currently identified as novel genes with ENSSSCG or LOC IDs were found to be coexpressed with annotated neighbouring transcripts in the same orientation, indicating they may be products of the same transcriptional unit. The meta-analytic approach to utilising public RNAseq data is extendable to include new data sets and new species and provides a framework to support the Functional Annotation of Animals Genomes (FAANG) initiative.

Kv2.1 mediates spatial and functional coupling of L-type calcium channels and ryanodine receptors in mammalian neurons

The voltage-gated K⁺ channel Kv2.1 serves a major structural role in the soma and proximal dendrites of mammalian brain neurons, tethering the plasma membrane (PM) to endoplasmic reticulum (ER). Although Kv2.1 clustering at neuronal ER-PM junctions (EPJs) is tightly regulated and highly conserved, its function remains unclear. By identifying and evaluating proteins in close spatial proximity to Kv2.1-containing EPJs, we discovered that a significant role of Kv2.1 at EPJs is to promote the clustering and functional coupling of PM L-type Ca²⁺ channels (LTCCs) to ryanodine receptor (RyR) ER Ca²⁺ release channels. Kv2.1 clustering also unexpectedly enhanced LTCC opening at polarized membrane potentials. This enabled Kv2.1-LTCC-RyR triads to generate localized Ca²⁺ release events (i.e., Ca²⁺ sparks) independently of action potentials. Together, these findings uncover a novel mode of LTCC regulation and establish a unique mechanism whereby Kv2.1-associated EPJs provide a molecular platform for localized somatodendritic Ca²⁺ signals in mammalian brain neurons.

Lrrc7 mutant mice model developmental emotional dysregulation that can be alleviated by mGluR5 allosteric modulation

LRRC7 has been identified as a candidate gene for severe childhood emotional dysregulation. Direct experimental evidence for a role of LRRC7 in the disease is needed, as is a better understanding of its impact on neuronal structure and signaling, and hence potential treatment targets. Here, we generated and analyzed an Lrrc7 mutant mouse line. Consistent with a critical role of LRRC7 in emotional regulation, mutant mice had inappropriate juvenile aggressive behavior and significant anxiety-like behavior and social dysfunction in adulthood. The pivotal role of mGluR5 signaling was demonstrated by rescue of behavioral defects with augmentation of mGluR5 receptor activity by 3-Cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB). Intra-peritoneal injection of CDPPB alleviated abnormal juvenile behavior, as well as anxiety-like behavior and hypersociability at adulthood. Furthermore, mutant primary neurons had impaired neurite outgrowth which was rescued by CDPPB treatment. In conclusion, Lrrc7 mutant mice provide a valuable tool to model childhood emotional dysregulation and persistent mental health comorbidities. Moreover, our data highlight an important role of LRRC7 in mGluR5 signaling, which is a potential new treatment target for anxiety and social dysfunction.

A Sam68-dependent alternative splicing program shapes postsynaptic protein complexes

Alternative splicing is one of the key mechanisms to increase the diversity of cellular transcriptomes, thereby expanding the coding capacity of the genome. This diversity is of particular importance in the nervous system with its elaborated cellular networks. Sam68, a member of the Signal Transduction Associated RNA-binding (STAR) family of RNA-binding proteins, is expressed in the developing and mature nervous system but its neuronal functions are poorly understood. Here, we perform genome-wide mapping of the Sam68-dependent alternative splicing program in mice. We find that Sam68 is required for the regulation of a set of alternative splicing events in pre-mRNAs encoding several postsynaptic scaffolding molecules that are central to the function of GABAergic and glutamatergic synapses. These components include Collybistin (Arhgef9), Gephyrin (Gphn), and Densin-180 (Lrrc7). Sam68-regulated Lrrc7 variants engage in differential protein interactions with signalling proteins, thus, highlighting a contribution of the Sam68 splicing program to shaping synaptic complexes. These findings suggest an important role for Sam68-dependent alternative splicing in the regulation of synapses in the central nervous system.

Distribution of densin in neurons

Densin is a scaffold protein known to associate with key elements of neuronal signaling. The present study examines the distribution of densin at the ultrastructural level in order to reveal potential sites that can support specific interactions of densin. Immunogold electron microscopy on hippocampal cultures shows intense labeling for densin at postsynaptic densities (PSDs), but also some labeling at extrasynaptic plasma membranes of soma and dendrites and endoplasmic reticulum. At the PSD, the median distance of label from the postsynaptic membrane was ~27 nm, with the majority of label (90%) confined within 40 nm from the postsynaptic membrane, indicating predominant localization of densin at the PSD core. Depolarization (90 mM K⁺ for 2 min) promoted a slight shift of densin label within the PSD complex resulting in 77% of label remaining within 40 nm from the postsynaptic membrane. Densin molecules firmly embedded within the PSD may target a minor pool of CaMKII to substrates at the PSD core.

Role of a conserved glutamine in the function of voltage-gated Ca2+ channels revealed by a mutation in human CACNA1D

Voltage-gated Ca_v Ca²⁺ channels play crucial roles in regulating gene transcription, neuronal excitability, and synaptic transmission. Natural or pathological variations in Ca_v channels have yielded rich insights into the molecular determinants controlling channel function. Here, we report the consequences of a natural, putatively disease-associated mutation in the CACNA1D gene encoding the pore-forming Ca_v1.3 α₁ subunit. The mutation causes a substitution of a glutamine residue that is highly conserved in the extracellular S1-S2 loop of domain II in all Ca_v channels with a histidine and was identified by whole-exome sequencing of an individual with moderate hearing impairment, developmental delay, and epilepsy. When introduced into the rat Ca_v1.3 cDNA, Q558H significantly decreased the density of Ca²⁺ currents in transfected HEK293T cells. Gating current analyses and cell-surface biotinylation experiments suggested that the smaller current amplitudes caused by Q558H were because of decreased numbers of functional Ca_v1.3 channels at the cell surface. The substitution also produced more sustained Ca²⁺ currents by weakening voltage-dependent inactivation. When inserted into the corresponding locus of Ca_v2.1, the substitution had similar effects as in Ca_v1.3. However, the substitution introduced in Ca_v3.1 reduced current density, but had no effects on voltage-dependent inactivation. Our results reveal a critical extracellular determinant of current density for all Ca_v family members and of voltage-dependent inactivation of Ca_v1.3 and Ca_v2.1 channels.

Dynamic L-type CaV1.2 channel trafficking facilitates CaV1.2 clustering and cooperative gating

L-type Ca_V1.2 channels are key regulators of gene expression, cell excitability and muscle contraction. Ca_V1.2 channels organize in clusters throughout the plasma membrane. This channel organization has been suggested to contribute to the concerted activation of adjacent Ca_V1.2 channels (e.g. cooperative gating). Here, we tested the hypothesis that dynamic intracellular and perimembrane trafficking of Ca_V1.2 channels is critical for formation and dissolution of functional channel clusters mediating cooperative gating. We found that Ca_V1.2 moves in vesicular structures of circular and tubular shape with diverse intracellular and submembrane trafficking patterns. Both microtubules and actin filaments are required for dynamic movement of Ca_V1.2 vesicles. These vesicles undergo constitutive homotypic fusion and fission events that sustain Ca_V1.2 clustering, channel activity and cooperative gating. Our study suggests that Ca_V1.2 clusters and activity can be modulated by diverse and unique intracellular and perimembrane vesicular dynamics to fine-tune Ca²⁺ signals.

Molecular mimicking of C-terminal phosphorylation tunes the surface dynamics of CaV1.2 calcium channels in hippocampal neurons

L-type voltage-gated Ca_V1.2 calcium channels (Ca_V1.2) are key regulators of neuronal excitability, synaptic plasticity, and excitation-transcription coupling. Surface-exposed Ca_V1.2 distributes in clusters along the dendrites of hippocampal neurons. A permanent exchange between stably clustered and laterally diffusive extra-clustered channels maintains steady-state levels of Ca_V1.2 at dendritic signaling domains. A dynamic equilibrium between anchored and diffusive receptors is a common feature among ion channels and is crucial to modulate signaling transduction. Despite the importance of this fine regulatory system, the molecular mechanisms underlying the surface dynamics of Ca_V1.2 are completely unexplored. Here, we examined the dynamic states of Ca_V1.2 depending on phosphorylation on Ser-1700 and Ser-1928 at the channel C terminus. Phosphorylation at these sites is strongly involved in Ca_V1.2-mediated nuclear factor of activated T cells (NFAT) signaling, long-term potentiation, and responsiveness to adrenergic stimulation. We engineered Ca_V1.2 constructs mimicking phosphorylation at Ser-1700 and Ser-1928 and analyzed their behavior at the membrane by immunolabeling protocols, fluorescence recovery after photobleaching, and single particle tracking. We found that the phosphomimetic S1928E variant increases the mobility of Ca_V1.2 without altering the steady-state maintenance of cluster in young neurons and favors channel stabilization later in differentiation. Instead, mimicking phosphorylation at Ser-1700 promoted the diffusive state of Ca_V1.2 irrespective of the differentiation stage. Together, these results reveal that phosphorylation could contribute to the establishment of channel anchoring mechanisms depending on the neuronal differentiation state. Finally, our findings suggest a novel mechanism by which phosphorylation at the C terminus regulates calcium signaling by tuning the content of Ca_V1.2 at signaling complexes.

Activity-Dependent Facilitation of CaV1.3 Calcium Channels Promotes KCa3.1 Activation in Hippocampal Neurons

Ca_V1 L-type calcium channels are key to regulating neuronal excitability, with the range of functional roles enhanced by interactions with calmodulin, accessory proteins, or CaMKII that modulate channel activity. In hippocampal pyramidal cells, a prominent elevation of Ca_V1 activity is apparent in late channel openings that can last for seconds following a depolarizing stimulus train. The current study tested the hypothesis that a reported interaction among Ca_V1.3 channels, the scaffolding protein densin, and CaMKII could generate a facilitation of channel activity that outlasts a depolarizing stimulus. We found that Ca_V1.3 but not Ca_V1.2 channels exhibit a long-duration calcium-dependent facilitation (L-CDF) that lasts up to 8 s following a brief 50 Hz stimulus train, but only when coexpressed with densin and CaMKII. To test the physiological role for Ca_V1.3 L-CDF, we coexpressed the intermediate-conductance KCa3.1 potassium channel, revealing a strong functional coupling to Ca_V1.3 channel activity that was accentuated by densin and CaMKII. Moreover, the Ca_V1.3-densin-CaMKII interaction gave rise to an outward tail current of up to 8 s duration following a depolarizing stimulus in both tsA-201 cells and male rat CA1 pyramidal cells. A slow afterhyperpolarization in pyramidal cells was reduced by a selective block of Ca_V1 channels by isradipine, a CaMKII blocker, and siRNA knockdown of densin, and spike frequency increased upon selective block of Ca_V1 channel conductance. The results are important in revealing a Ca_V1.3-densin-CaMKII interaction that extends the contribution of Ca_V1.3 calcium influx to a time frame well beyond a brief input train.SIGNIFICANCE STATEMENT Ca_V1 L-type calcium channels play a key role in regulating the output of central neurons by providing calcium influx during repetitive inputs. This study identifies a long-duration calcium-dependent facilitation (L-CDF) of Ca_V1.3 channels that depends on the scaffolding protein densin and CaMKII and that outlasts a depolarizing stimulus by seconds. We further show a tight functional coupling between Ca_V1.3 calcium influx and the intermediate-conductance KCa3.1 potassium channel that promotes an outward tail current of up to 8 s following a depolarizing stimulus. Tests in CA1 hippocampal pyramidal cells reveal that a slow AHP is reduced by blocking different components of the Ca_V1.3-densin-CaMKII interaction, identifying an important role for Ca_V1.3 L-CDF in regulating neuronal excitability.

α-Actinin Promotes Surface Localization and Current Density of the Ca2+ Channel CaV1.2 by Binding to the IQ Region of the α1 Subunit

The voltage-gated L-type Ca²⁺ channel Ca_V1.2 is crucial for initiating heartbeat and control of a number of neuronal functions such as neuronal excitability and long-term potentiation. Mutations of Ca_V1.2 subunits result in serious health problems, including arrhythmia, autism spectrum disorders, immunodeficiency, and hypoglycemia. Thus, precise control of Ca_V1.2 surface expression and localization is essential. We previously reported that α-actinin associates and colocalizes with neuronal Ca_V1.2 channels and that shRNA-mediated depletion of α-actinin significantly reduces localization of endogenous Ca_V1.2 in dendritic spines in hippocampal neurons. Here we investigated the hypothesis that direct binding of α-actinin to Ca_V1.2 supports its surface expression. Using two-hybrid screens and pull-down assays, we identified three point mutations (K1647A, Y1649A, and I1654A) in the central, pore-forming α₁1.2 subunit of Ca_V1.2 that individually impaired α-actinin binding. Surface biotinylation and flow cytometry assays revealed that Ca_V1.2 channels composed of the corresponding α-actinin-binding-deficient mutants result in a 35-40% reduction in surface expression compared to that of wild-type channels. Moreover, the mutant Ca_V1.2 channels expressed in HEK293 cells exhibit a 60-75% decrease in current density. The larger decrease in current density as compared to surface expression imparted by these α₁1.2 subunit mutations hints at the possibility that α-actinin not only stabilizes surface localization of Ca_V1.2 but also augments its ion conducting activity.