Erbin enhances voltage-dependent facilitation of Ca(v)1.3 Ca2+ channels through relief of an autoinhibitory domain in the Ca(v)1.3 alpha1 subunit

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.

The leucine-rich repeat signaling scaffolds Shoc2 and Erbin: cellular mechanism and role in disease

Leucine-rich repeat-containing proteins (LRR proteins) are involved in supporting a large number of cellular functions. In this review, we summarize recent advancements in understanding functions of the LRR proteins as signaling scaffolds. In particular, we explore what we have learned about the mechanisms of action of the LRR scaffolds Shoc2 and Erbin and their roles in normal development and disease. We discuss Shoc2 and Erbin in the context of their multiple known interacting partners in various cellular processes and summarize often unexpected functions of these proteins through analysis of their roles in human pathologies. We also review these LRR scaffold proteins as promising therapeutic targets and biomarkers with potential application across various pathologies.

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.

Single-Channel Resolution of the Interaction between C-Terminal CaV1.3 Isoforms and Calmodulin

Voltage-dependent calcium (Ca_V) 1.3 channels are involved in the control of cellular excitability and pacemaking in neuronal, cardiac, and sensory cells. Various proteins interact with the alternatively spliced channel C-terminus regulating gating of Ca_V1.3 channels. Binding of a regulatory calcium-binding protein calmodulin (CaM) to the proximal C-terminus leads to the boosting of channel activity and promotes calcium-dependent inactivation (CDI). The C-terminal modulator domain (CTM) of Ca_V1.3 channels can interfere with the CaM binding, thereby inhibiting channel activity and CDI. Here, we compared single-channel gating behavior of two natural Ca_V1.3 splice isoforms: the long Ca_V1.3₄₂ with the full-length CTM and the short Ca_V1.3_42A with the C-terminus truncated before the CTM. We found that CaM regulation of Ca_V1.3 channels is dynamic on a minute timescale. We observed that at equilibrium, single Ca_V1.3₄₂ channels occasionally switched from low to high open probability, which perhaps reflects occasional binding of CaM despite the presence of CTM. Similarly, when the amount of the available CaM in the cell was reduced, the short Ca_V1.3_42A isoform showed patterns of the low channel activity. CDI also underwent periodic changes with corresponding kinetics in both isoforms. Our results suggest that the competition between CTM and CaM is influenced by calcium, allowing further fine-tuning of Ca_V1.3 channel activity for particular cellular needs.

Enhancing VTA Cav1.3 L-type Ca2+ channel activity promotes cocaine and mood-related behaviors via overlapping AMPA receptor mechanisms in the nucleus accumbens

Genetic factors significantly influence susceptibility for substance abuse and mood disorders. Rodent studies have begun to elucidate a role of Cav1.3 L-type Ca2+ channels in neuropsychiatric-related behaviors, such as addictive and depressive-like behaviors. Human studies have also linked the CACNA1D gene, which codes for the Cav1.3 protein, with bipolar disorder. However, the neurocircuitry and the molecular mechanisms underlying the role of Cav1.3 in neuropsychiatric phenotypes are not well established. In the present study, we directly manipulated Cav1.3 channels in Cav1.2 dihydropyridine insensitive mutant mice and found that ventral tegmental area (VTA) Cav1.3 channels mediate cocaine-related and depressive-like behavior through a common nucleus accumbens (NAc) shell calcium-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (CP-AMPAR) mechanism that requires GluA1 phosphorylation at S831. Selective activation of VTA Cav1.3 with (±)-BayK-8644 (BayK) enhanced cocaine conditioned place preference and cocaine psychomotor activity while inducing depressive-like behavior, an effect not observed in S831A phospho-mutant mice. Infusion of the CP-AMPAR-specific blocker Naspm into the NAc shell reversed the cocaine and depressive-like phenotypes. In addition, activation of VTA Cav1.3 channels resulted in social behavioral deficits. In contrast to the cocaine- and depression-related phenotypes, GluA1/A2 AMPARs in the NAc core mediated social deficits, independent of S831-GluA1 phosphorylation. Using a candidate gene analysis approach, we also identified single-nucleotide polymorphisms in the CACNA1D gene associated with cocaine dependence in human subjects. Together, our findings reveal novel, overlapping mechanisms through which VTA Cav1.3 mediates cocaine-related, depressive-like and social phenotypes, suggesting that Cav1.3 may serve as a target for the treatment of neuropsychiatric symptoms.

Rab Interacting Molecules 2 and 3 Directly Interact with the Pore-Forming CaV1.3 Ca2+ Channel Subunit and Promote Its Membrane Expression

Rab interacting molecules (RIMs) are multi-domain proteins that positively regulate the number of Ca²⁺ channels at the presynaptic active zone (AZ). Several molecular mechanisms have been demonstrated for RIM-binding to components of the presynaptic Ca²⁺ channel complex, the key signaling element at the AZ. Here, we report an interaction of the C₂B domain of RIM2α and RIM3γ with the C-terminus of the pore-forming α-subunit of Ca_V1.3 channels (Ca_V1.3α1), which mediate stimulus-secretion coupling at the ribbon synapses of cochlear inner hair cells (IHCs). Co-expressing full-length RIM2α with a Ca²⁺ channel complex closely resembling that of IHCs (Ca_V1.3α1-Ca_Vß2a) in HEK293 cells doubled the Ca²⁺-current and shifted the voltage-dependence of Ca²⁺ channel activation by approximately +3 mV. Co-expression of the short RIM isoform RIM3γ increased the Ca_V1.3α1-Ca_Vß2a-mediated Ca²⁺-influx in HEK293 cells, but disruption of RIM3γ in mice left Ca²⁺-influx in IHCs and hearing intact. In conclusion, we propose that RIM2α and RIM3γ directly interact with the C-terminus of the pore-forming subunit of Ca_V1.3 Ca²⁺ channels and positively regulate their plasma membrane expression in HEK293 cells.

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

Voltage-gated Ca_v1.2 and Ca_v1.3 (L-type) Ca²⁺ channels regulate neuronal excitability, synaptic plasticity, and learning and memory. Densin-180 (densin) is an excitatory synaptic protein that promotes Ca²⁺-dependent facilitation of voltage-gated Ca_v1.3 Ca²⁺ channels in transfected cells. Mice lacking densin (densin KO) exhibit defects in synaptic plasticity, spatial memory, and increased anxiety-related behaviors-phenotypes that more closely match those in mice lacking Ca_v1.2 than Ca_v1.3. Therefore, we investigated the functional impact of densin on Ca_v1.2. We report that densin is an essential regulator of Ca_v1.2 in neurons, but has distinct modulatory effects compared with its regulation of Ca_v1.3. Densin binds to the N-terminal domain of Ca_v1.2, but not that of Ca_v1.3, and increases Ca_v1.2 currents in transfected cells and in neurons. In transfected cells, densin accelerates the forward trafficking of Ca_v1.2 channels without affecting their endocytosis. Consistent with a role for densin in increasing the number of postsynaptic Ca_v1.2 channels, overexpression of densin increases the clustering of Ca_v1.2 in dendrites of hippocampal neurons in culture. Compared with wild-type mice, the cell surface levels of Ca_v1.2 in the brain, as well as Ca_v1.2 current density and signaling to the nucleus, are reduced in neurons from densin KO mice. We conclude that densin is an essential regulator of neuronal Ca_v1 channels and ensures efficient Ca_v1.2 Ca²⁺ signaling at excitatory synapses.SIGNIFICANCE STATEMENT The number and localization of voltage-gated Ca_v Ca²⁺ channels are crucial determinants of neuronal excitability and synaptic transmission. We report that the protein densin-180 is highly enriched at excitatory synapses in the brain and enhances the cell surface trafficking and postsynaptic localization of Ca_v1.2 L-type Ca²⁺ channels in neurons. This interaction promotes coupling of Ca_v1.2 channels to activity-dependent gene transcription. Our results reveal a mechanism that may contribute to the roles of Ca_v1.2 in regulating cognition and mood.

Motor Neurons

Motor neurons translate synaptic input from widely distributed premotor networks into patterns of action potentials that orchestrate motor unit force and motor behavior. Intercalated between the CNS and muscles, motor neurons add to and adjust the final motor command. The identity and functional properties of this facility in the path from synaptic sites to the motor axon is reviewed with emphasis on voltage sensitive ion channels and regulatory metabotropic transmitter pathways. The catalog of the intrinsic response properties, their underlying mechanisms, and regulation obtained from motoneurons in in vitro preparations is far from complete. Nevertheless, a foundation has been provided for pursuing functional significance of intrinsic response properties in motoneurons in vivo during motor behavior at levels from molecules to systems. © 2017 American Physiological Society. Compr Physiol 7:463-484, 2017.

Splice variants of the CaV1.3 L-type calcium channel regulate dendritic spine morphology

Dendritic spines are the postsynaptic compartments of glutamatergic synapses in the brain. Their number and shape are subject to change in synaptic plasticity and neurological disorders including autism spectrum disorders and Parkinson’s disease. The L-type calcium channel Ca_V1.3 constitutes an important calcium entry pathway implicated in the regulation of spine morphology. Here we investigated the importance of full-length Ca_V1.3_L and two C-terminally truncated splice variants (Ca_V1.3_42A and Ca_V1.3_43S) and their modulation by densin-180 and shank1b for the morphology of dendritic spines of cultured hippocampal neurons. Live-cell immunofluorescence and super-resolution microscopy of epitope-tagged Ca_V1.3_L revealed its localization at the base-, neck-, and head-region of dendritic spines. Expression of the short splice variants or deletion of the C-terminal PDZ-binding motif in Ca_V1.3_L induced aberrant dendritic spine elongation. Similar morphological alterations were induced by co-expression of densin-180 or shank1b with Ca_V1.3_L and correlated with increased Ca_V1.3 currents and dendritic calcium signals in transfected neurons. Together, our findings suggest a key role of Ca_V1.3 in regulating dendritic spine structure. Under physiological conditions it may contribute to the structural plasticity of glutamatergic synapses. Conversely, altered regulation of Ca_V1.3 channels may provide an important mechanism in the development of postsynaptic aberrations associated with neurodegenerative disorders.

Regulation of Postsynaptic Stability by the L-type Calcium Channel CaV1.3 and its Interaction with PDZ Proteins

Alterations in dendritic spine morphology and postsynaptic structure are a hallmark of neurological disorders. Particularly spine pruning of striatal medium spiny neurons and aberrant rewiring of corticostriatal synapses have been associated with the pathology of Parkinson’s disease and L-DOPA induced dyskinesia, respectively. Owing to its low activation threshold the neuronal L-type calcium channel Ca_V1.3 is particularly critical in the control of neuronal excitability and thus in the calcium-dependent regulation of neuronal functions. Ca_V1.3 channels are located in dendritic spines and contain a C-terminal class 1 PDZ domain-binding sequence. Until today the postsynaptic PDZ domain proteins shank, densin-180, and erbin have been shown to interact with Ca_V1.3 channels and to modulate their current properties. Interestingly experimental evidence suggests an involvement of all three PDZ proteins as well as Ca_V1.3 itself in regulating dendritic and postsynaptic morphology. Here we briefly review the importance of Ca_V1.3 and its proposed interactions with PDZ proteins for the stability of dendritic spines. With a special focus on the pathology associated with Parkinson’s disease, we discuss the hypothesis that Ca_V1.3 L-type calcium channels may be critical modulators of dendritic spine stability.

Characterization of C-terminal Splice Variants of Cav1.4 Ca2+ Channels in Human Retina

Voltage-gated Ca(2+) channels (Cav) undergo extensive alternative splicing that greatly enhances their functional diversity in excitable cells. Here, we characterized novel splice variants of the cytoplasmic C-terminal domain of Cav1.4 Ca(2+) channels that regulate neurotransmitter release in photoreceptors in the retina. These variants lack a portion of exon 45 and/or the entire exon 47 (Cav1.4Δex p45, Cav1.4Δex 47, Cav1.4Δex p45,47) and are expressed in the retina of primates but not mice. Although the electrophysiological properties of Cav1.4Δex p45 are similar to those of full-length channels (Cav1.4FL), skipping of exon 47 dramatically alters Cav1.4 function. Deletion of exon 47 removes part of a C-terminal automodulatory domain (CTM) previously shown to suppress Ca(2+)-dependent inactivation (CDI) and to cause a positive shift in the voltage dependence of channel activation. Exon 47 is crucial for these effects of the CTM because variants lacking this exon show intense CDI and activate at more hyperpolarized voltages than Cav1.4FL The robust CDI of Cav1.4Δex 47 is suppressed by CaBP4, a regulator of Cav1.4 channels in photoreceptors. Although CaBP4 enhances activation of Cav1.4FL, Cav1.4Δex 47 shows similar voltage-dependent activation in the presence and absence of CaBP4. We conclude that exon 47 encodes structural determinants that regulate CDI and voltage-dependent activation of Cav1.4, and is necessary for modulation of channel activation by CaBP4.

Cell-type-specific tuning of Cav1.3 Ca(2+)-channels by a C-terminal automodulatory domain

Cav1.3 L-type Ca(2+)-channel function is regulated by a C-terminal automodulatory domain (CTM). It affects channel binding of calmodulin and thereby tunes channel activity by interfering with Ca(2+)- and voltage-dependent gating. Alternative splicing generates short C-terminal channel variants lacking the CTM resulting in enhanced Ca(2+)-dependent inactivation and stronger voltage-sensitivity upon heterologous expression. However, the role of this modulatory domain for channel function in its native environment is unkown. To determine its functional significance in vivo, we interrupted the CTM with a hemagglutinin tag in mutant mice (Cav1.3DCRD(HA/HA)). Using these mice we provide biochemical evidence for the existence of long (CTM-containing) and short (CTM-deficient) Cav1.3 α1-subunits in brain. The long (HA-labeled) Cav1.3 isoform was present in all ribbon synapses of cochlear inner hair cells. CTM-elimination impaired Ca(2+)-dependent inactivation of Ca(2+)-currents in hair cells but increased it in chromaffin cells, resulting in hyperpolarized resting potentials and reduced pacemaking. CTM disruption did not affect hearing thresholds. We show that the modulatory function of the CTM is affected by its native environment in different cells and thus occurs in a cell-type specific manner in vivo. It stabilizes gating properties of Cav1.3 channels required for normal electrical excitability.

The role of auxiliary subunits for the functional diversity of voltage-gated calcium channels

Voltage-gated calcium channels (VGCCs) represent the sole mechanism to convert membrane depolarization into cellular functions like secretion, contraction, or gene regulation. VGCCs consist of a pore-forming α₁ subunit and several auxiliary channel subunits. These subunits come in multiple isoforms and splice-variants giving rise to a stunning molecular diversity of possible subunit combinations. It is generally believed that specific auxiliary subunits differentially regulate the channels and thereby contribute to the great functional diversity of VGCCs. If auxiliary subunits can associate and dissociate from pre-existing channel complexes, this would allow dynamic regulation of channel properties. However, most auxiliary subunits modulate current properties very similarly, and proof that any cellular calcium channel function is indeed modulated by the physiological exchange of auxiliary subunits is still lacking. In this review we summarize available information supporting a differential modulation of calcium channel functions by exchange of auxiliary subunits, as well as experimental evidence in support of alternative functions of the auxiliary subunits. At the heart of the discussion is the concept that, in their native environment, VGCCs function in the context of macromolecular signaling complexes and that the auxiliary subunits help to orchestrate the diverse protein–protein interactions found in these calcium channel signalosomes. Thus, in addition to a putative differential modulation of current properties, differential subcellular targeting properties and differential protein–protein interactions of the auxiliary subunits may explain the need for their vast molecular diversity. J. Cell. Physiol. 999: 00–00, 2015. © 2015 The Authors. Journal of Cellular Physiology Published by Wiley Periodicals, Inc. J. Cell. Physiol. 230: 2019–2031, 2015. © 2015 Wiley Periodicals, Inc.

The PDZ motif of the α1C subunit is not required for surface trafficking and adrenergic modulation of CaV1.2 channel in the heart

Voltage-gated Ca(2+) channels play a key role in initiating muscle excitation-contraction coupling, neurotransmitter release, gene expression, and hormone secretion. The association of CaV1.2 with a supramolecular complex impacts trafficking, localization, turnover, and, most importantly, multifaceted regulation of its function in the heart. Several studies hint at an important role for the C terminus of the α1C subunit as a hub for multidimensional regulation of CaV1.2 channel trafficking and function. Recent studies have demonstrated an important role for the four-residue PDZ binding motif at the C terminus of α1C in interacting with scaffold proteins containing PDZ domains, in the subcellular localization of CaV1.2 in neurons, and in the efficient signaling to cAMP-response element-binding protein in neurons. However, the role of the α1C PDZ ligand domain in the heart is not known. To determine whether the α1C PDZ motif is critical for CaV1.2 trafficking and function in cardiomyocytes, we generated transgenic mice with inducible expression of an N-terminal FLAG epitope-tagged dihydropyridine-resistant α1C with the PDZ motif deleted (ΔPDZ). These mice were crossed with α-myosin heavy chain reverse transcriptional transactivator transgenic mice, and the double-transgenic mice were fed doxycycline. The ΔPDZ channels expressed, trafficked to the membrane, and supported robust excitation-contraction coupling in the presence of nisoldipine, a dihydropyridine Ca(2+) channel blocker, providing functional evidence that they appropriately target to dyads. The ΔPDZ Ca(2+) channels were appropriately regulated by isoproterenol and forskolin. These data indicate that the α1C PDZ motif is not required for surface trafficking, localization to the dyad, or adrenergic stimulation of CaV1.2 in adult cardiomyocytes.

L-type Ca2+ channels in heart and brain

L-type calcium channels (Cav1) represent one of the three major classes (Cav1–3) of voltage-gated calcium channels. They were identified as the target of clinically used calcium channel blockers (CCBs; so-called calcium antagonists) and were the first class accessible to biochemical characterization. Four of the 10 known α1 subunits (Cav1.1–Cav1.4) form the pore of L-type calcium channels (LTCCs) and contain the high-affinity drug-binding sites for dihydropyridines and other chemical classes of organic CCBs. In essentially all electrically excitable cells one or more of these LTCC isoforms is expressed, and therefore it is not surprising that many body functions including muscle, brain, endocrine, and sensory function depend on proper LTCC activity. Gene knockouts and inherited human diseases have allowed detailed insight into the physiological and pathophysiological role of these channels. Genome-wide association studies and analysis of human genomes are currently providing even more hints that even small changes of channel expression or activity may be associated with disease, such as psychiatric disease or cardiac arrhythmias. Therefore, it is important to understand the structure–function relationship of LTCC isoforms, their differential contribution to physiological function, as well as their fine-tuning by modulatory cellular processes.

Harmonin enhances voltage-dependent facilitation of Cav1.3 channels and synchronous exocytosis in mouse inner hair cells

Cav1.3 channels mediate Ca(2+) influx that triggers exocytosis of glutamate at cochlear inner hair cell (IHC) synapses. Harmonin is a PDZ-domain-containing protein that interacts with the C-terminus of the Cav1.3 α1 subunit (α11.3) and controls cell surface Cav1.3 levels by promoting ubiquitin-dependent proteosomal degradation. However, PDZ-domain-containing proteins have diverse functions and regulate other Cav1.3 properties, which could collectively influence presynaptic transmitter release. Here, we report that harmonin binding to the α11.3 distal C-terminus (dCT) enhances voltage-dependent facilitation (VDF) of Cav1.3 currents both in transfected HEK293T cells and in mouse inner hair cells. In HEK293T cells, this effect of harmonin was greater for Cav1.3 channels containing the auxiliary Cav β1 than with the β2 auxiliary subunit. Cav1.3 channels lacking the α11.3 dCT were insensitive to harmonin modulation. Moreover, the 'deaf-circler' dfcr mutant form of harmonin, which does not interact with the α11.3 dCT, did not promote VDF. In mature IHCs from mice expressing the dfcr harmonin mutant, Cav1.3 VDF was less than in control IHCs. This difference was not observed between control and dfcr IHCs prior to hearing onset. Membrane capacitance recordings from dfcr IHCs revealed a role for harmonin in synchronous exocytosis and in increasing the efficiency of Ca(2+) influx for triggering exocytosis. Collectively, our results indicate a multifaceted presynaptic role of harmonin in IHCs in regulating Cav1.3 Ca(2+) channels and exocytosis.

Distinct localization and modulation of Cav1.2 and Cav1.3 L-type Ca2+ channels in mouse sinoatrial node

Dysregulation of L-type Ca(2+) currents in sinoatrial nodal (SAN) cells causes cardiac arrhythmia. Both Ca(v)1.2 and Ca(v)1.3 channels mediate sinoatrial L-type currents. Whether these channels exhibit differences in modulation and localization, which could affect their contribution to pacemaking, is unknown. In this study, we characterized voltage-dependent facilitation (VDF) and subcellular localization of Ca(v)1.2 and Ca(v)1.3 channels in mouse SAN cells and determined how these properties of Ca(v)1.3 affect sinoatrial pacemaking in a mathematical model. Whole cell Ba(2+) currents were recorded from SAN cells from mice carrying a point mutation that renders Ca(v)1.2 channels relatively insensitive to dihydropyridine antagonists. The Ca(v)1.2-mediated current was isolated in the presence of nimodipine (1 μm), which was subtracted from the total current to yield the Ca(v)1.3 component. With strong depolarizations (+80 mV), Ca(v)1.2 underwent significantly stronger inactivation than Ca(v)1.3. VDF of Ca(v)1.3 was evident during recovery from inactivation at a time when Ca(v)1.2 remained inactivated. By immunofluorescence, Ca(v)1.3 colocalized with ryanodine receptors in sarcomeric structures while Ca(v)1.2 was largely restricted to the delimiting plasma membrane. Ca(v)1.3 VDF enhanced recovery of pacemaker activity after pauses and positively regulated pacemaking during slow heart rate in a numerical model of mouse SAN automaticity, including preferential coupling of Ca(v)1.3 to ryanodine receptor-mediated Ca(2+) release. We conclude that strong VDF and colocalization with ryanodine receptors in mouse SAN cells are unique properties that may underlie a specific role for Ca(v)1.3 in opposing abnormal slowing of heart rate.

Association of shank 1A scaffolding protein with cone photoreceptor terminals in the mammalian retina

Photoreceptor terminals contain post-synaptic density (PSD) proteins e.g., PSD-95/PSD-93, but their role at photoreceptor synapses is not known. PSDs are generally restricted to post-synaptic boutons in central neurons and form scaffolding with multiple proteins that have structural and functional roles in neuronal signaling. The Shank family of proteins (Shank 1–3) functions as putative anchoring proteins for PSDs and is involved in the organization of cytoskeletal/signaling complexes in neurons. Specifically, Shank 1 is restricted to neurons and interacts with both receptors and signaling molecules at central neurons to regulate plasticity. However, it is not known whether Shank 1 is expressed at photoreceptor terminals. In this study we have investigated Shank 1A localization in the outer retina at photoreceptor terminals. We find that Shank 1A is expressed presynaptically in cone pedicles, but not rod spherules, and it is absent from mice in which the Shank 1 gene is deleted. Shank 1A co-localizes with PSD-95, peanut agglutinin, a marker of cone terminals, and glycogen phosphorylase, a cone specific marker. These findings provide convincing evidence for Shank 1A expression in both the inner and outer plexiform layers, and indicate a potential role for PSD-95/Shank 1 complexes at cone synapses in the outer retina.

The expression of whirlin and Cav1.3α₁ is mutually independent in photoreceptors

Whirlin is a gene responsible for Usher syndrome type II (USH2) and congenital deafness. In photoreceptors, it organizes a protein complex through binding to proteins encoded by other USH2 genes, usherin (USH2A) and G-protein-coupled receptor 98 (GPR98). Recently, Ca(v)1.3α(1) (α(1D)) has been discovered to interact with whirlin in vitro and these two proteins are localized to the same subcellular compartments in photoreceptors. Accordingly, it is proposed that Ca(v)1.3α(1) is in the USH2 protein complex and that the USH2 protein complex is involved in regulating Ca(2+) in photoreceptors. To test this hypothesis, we investigated the interdependence of Ca(v)1.3α(1) and whirlin expression in photoreceptors. We found that lack of Ca(v)1.3α(1) did not change the whirlin distribution or expression level in photoreceptors. In the retina, several Ca(v)1.3α(1) splice variants were found at the RNA level. Among them, the whirlin-interacting Ca(v)1.3α(1) long variant had no change in its protein expression level in the absence of whirlin. The localization of Ca(v)1.3α(1) in photoreceptors, published previously, cannot be confirmed. Therefore, the mutual independence of whirlin and Ca(v)1.3α(1) expressions in photoreceptors suggests that Ca(v)1.3α(1) may not be a key member of the USH2 protein complex at the periciliary membrane complex.

Structural determinants of CaV1.3 L-type calcium channel gating

A C-terminal modulatory domain (CTM) tightly regulates the biophysical properties of Ca(v)1.3 L-type Ca(2+) channels, in particular the voltage dependence of activation (V(0.5)) and Ca(2+) dependent inactivation (CDI). A functional CTM is present in the long C-terminus of human and mouse Ca(v)1.3 (Ca(v)1.3(L)), but not in a rat long cDNA clone isolated from superior cervical ganglia neurons (rCa(v)1.3(scg)). We therefore addressed the question if this represents a species-difference and compared the biophysical properties of rCa(v)1.3(scg) with a rat cDNA isolated from rat pancreas (rCa(v)1.3(L)). When expressed in tsA-201 cells under identical experimental conditions rCa(v)1.3(L) exhibited Ca(2+) current properties indistinguishable from human and mouse Ca(v)1.3(L), compatible with the presence of a functional CTM. In contrast, rCa(v)1.3(scg) showed gating properties similar to human short splice variants lacking a CTM. rCa(v)1.3(scg) differs from rCa(v)1.3(L) at three single amino acid (aa) positions, one alternative spliced exon (exon31), and a N-terminal polymethionine stretch with two additional lysines. Two aa (S244, A2075) in rCa(v)1.3(scg) explained most of the functional differences to rCa(v)1.3(L). Their mutation to the corresponding residues in rCa(v)1.3(L) (G244, V2075) revealed that both contributed to the more negative V 0.5, but caused opposite effects on CDI. A2075 (located within a region forming the CTM) additionally permitted higher channel open probability. The cooperative action in the double-mutant restored gating properties similar to rCa(v)1.3(L). We found no evidence for transcripts containing one of the single rCa(v)1.3(scg) mutations in rat superior cervical ganglion preparations. However, the rCa(v)1.3(scg) variant provided interesting insight into the structural machinery involved in Ca(v)1.3 gating.

ERBIN is a new SARA-interacting protein: competition between SARA and SMAD2 and SMAD3 for binding to ERBIN

SARA, an early endosomal protein, plays a key role in TGFβ signalling, as it presents SMAD2 and SMAD3 for phosphorylation by the activated TGFβ receptors. Here, we show that ERBIN is a new SARA-interacting protein that can be recruited by SARA to early endosomes. ERBIN was recently shown to bind and segregate phosphorylated SMAD2 and SMAD3 (SMAD2/3) in the cytoplasm, thereby inhibiting SMAD2/3-dependent transcription. SARA binds to ERBIN using a new domain, which we have called the ERBID (ERBIN-binding domain), whereas ERBIN binds to SARA using a domain (amino acids 1208-1265) that also interacts with SMAD2 and SMAD3, which we have called the SSID (SARA- and SMAD-interacting domain). We additionally show that SARA competes with SMAD2/3 for binding to ERBIN. In agreement, overexpression of SARA or the ERBID peptide reverses the inhibitory effect of ERBIN on SMAD2/3-dependent transcription. Taken together, these data suggest that the response of cells to TGFβ and activin A can be influenced by the relative concentrations of SARA, ERBIN and SMAD2/3.

Harmonin inhibits presynaptic Cav1.3 Ca²⁺ channels in mouse inner hair cells

Harmonin is a scaffolding protein required for normal mechanosensory function in hair cells. Here, we describe a novel presynaptic association of harmonin and Ca_v1.3 Ca²⁺ channels at the mouse inner hair cell synapse, which limits channel availability through a ubiquitin-dependent pathway.

Nrg1/ErbB signaling networks in Schwann cell development and myelination

Neuregulin-1 (Nrg1) provides a key axonal signal that regulates Schwann cell proliferation, migration and myelination through binding to ErbB2/3 receptors. The analysis of a number of genetic models has unmasked fundamental mechanisms underlying the specificity of the Nrg1/ErbB signaling axis. Differential expression of Nrg1 isoforms, Nrg1 processing, and ErbB receptor localization and trafficking represent important regulatory themes in the control of Nrg1/ErbB function. Nrg1 binding to ErbB2/3 receptors results in the activation of intracellular signal transduction pathways that initiate changes in Schwann cell behavior. Here, we review data that has defined the role of key Nrg1/ErbB signaling components like Shp2, ERK1/2, FAK, Rac1/Cdc42 and calcineurin in development of the Schwann cell lineage in vivo. Many of these regulators receive converging signals from other cues that are provided by Notch, integrin or G-protein coupled receptors. Signaling by multiple extracellular factors may act as key modifiers and allow Schwann cells at different developmental stages to respond in distinct manners to the Nrg1/ErbB signal.

Channelopathies in Cav1.1, Cav1.3, and Cav1.4 voltage-gated L-type Ca2+ channels

Voltage-gated Ca2+ channels couple membrane depolarization to Ca2+-dependent intracellular signaling events. This is achieved by mediating Ca2+ ion influx or by direct conformational coupling to intracellular Ca2+ release channels. The family of Cav1 channels, also termed L-type Ca2+ channels (LTCCs), is uniquely sensitive to organic Ca2+ channel blockers and expressed in many electrically excitable tissues. In this review, we summarize the role of LTCCs for human diseases caused by genetic Ca2+ channel defects (channelopathies). LTCC dysfunction can result from structural aberrations within their pore-forming alpha1 subunits causing hypokalemic periodic paralysis and malignant hyperthermia sensitivity (Cav1.1 alpha1), incomplete congenital stationary night blindness (CSNB2; Cav1.4 alpha1), and Timothy syndrome (Cav1.2 alpha1; reviewed separately in this issue). Cav1.3 alpha1 mutations have not been reported yet in humans, but channel loss of function would likely affect sinoatrial node function and hearing. Studies in mice revealed that LTCCs indirectly also contribute to neurological symptoms in Ca2+ channelopathies affecting non-LTCCs, such as Cav2.1 alpha1 in tottering mice. Ca2+ channelopathies provide exciting disease-related molecular detail that led to important novel insight not only into disease pathophysiology but also to mechanisms of channel function.

Ca2+-dependent facilitation of Cav1.3 Ca2+ channels by densin and Ca2+/calmodulin-dependent protein kinase II

Ca(v)1 (L-type) channels and calmodulin-dependent protein kinase II (CaMKII) are key regulators of Ca(2+) signaling in neurons. CaMKII directly potentiates the activity of Ca(v)1.2 and Ca(v)1.3 channels, but the underlying molecular mechanisms are incompletely understood. Here, we report that the CaMKII-associated protein densin is required for Ca(2+)-dependent facilitation of Ca(v)1.3 channels. While neither CaMKII nor densin independently affects Ca(v)1.3 properties in transfected HEK293T cells, the two together augment Ca(v)1.3 Ca(2+) currents during repetitive, but not sustained, depolarizing stimuli. Facilitation requires Ca(2+), CaMKII activation, and its association with densin, as well as densin binding to the Ca(v)1.3 alpha(1) subunit C-terminal domain. Ca(v)1.3 channels and densin are targeted to dendritic spines in neurons and form a complex with CaMKII in the brain. Our results demonstrate a novel mechanism for Ca(2+)-dependent facilitation that may intensify postsynaptic Ca(2+) signals during high-frequency stimulation.

PDZ-domain containing-2 (PDZD2) drives the maturity of human fetal pancreatic progenitor-derived islet-like cell clusters with functional responsiveness against membrane depolarization

We recently reported the isolation and characterization of a population of pancreatic progenitor cells (PPCs) from early trimester human fetal pancreata. The PPCs, being the forerunners of adult pancreatic cell lineages, were amenable to growth and differentiation into insulin-secreting islet-like cell clusters (ICCs) upon stimulation by adequate morphogens. Of note, a novel morphogenic factor, PDZ-domain containing-2 (PDZD2) and its secreted form (sPDZD2) were ubiquitously expressed in the PPCs. Our goals for this study were to evaluate the potential role of sPDZD2 in stimulating PPC differentiation and to establish the optimal concentration for such stimulation. We found that 10(-9)M sPDZD2 promoted PPC differentiation, as evidenced by the upregulation of the pancreatic endocrine markers (PDX-1, NGN3, NEURO-D, ISL-1, NKX 2.2, NKX 6.1) and INSULIN mRNA. Inhibited endogenous production of sPDZD2 suppressed expression of these factors. Secreted PDZD2 treatment significantly elevated the C-peptide content of the ICCs and increased the basal rate of insulin secretion. However, they remained unresponsive to glucose stimulation, reflected by a minimal increase in GLUT-2 and GLUCOKINASE mRNA expression. Interestingly, sPDZD2 treatment induced increased expression of the L-type voltage-gated calcium channel (Ca(v)1.2) in the ICCs, triggering calcium ion influx under KCl stimulation and conferring an ability to secrete insulin in response to KCl. Pancreatic progenitor cells from 10- and 13-week fetal pancreata showed peak expression of endogenous sPDZD2, implying that sPDZD2 has a specific role in islet development during the first trimester. In conclusion, our data suggest that sPDZD2 promotes functional maturation of human fetal PPC-derived ICCs, thus enhancing its transplanting potentials.

The origin and evolution of synapses

Understanding the evolutionary origins of behaviour is a central aim in the study of biology and may lead to insights into human disorders. Synaptic transmission is observed in a wide range of invertebrate and vertebrate organisms and underlies their behaviour. Proteomic studies of the molecular components of the highly complex mammalian postsynaptic machinery point to an ancestral molecular machinery in unicellular organisms--the protosynapse--that existed before the evolution of metazoans and neurons, and hence challenges existing views on the origins of the brain. The phylogeny of the molecular components of the synapse provides a new model for studying synapse diversity and complexity, and their implications for brain evolution.

Erbin controls dendritic morphogenesis by regulating localization of delta-catenin

The LAP [leucine-rich and postsynaptic density-95/Discs large/zona occludens-1 (PDZ)] protein erbin and delta-catenin, a component of the cadherin-catenin cell adhesion complex, are highly expressed in neurons and associate through PDZ-mediated interaction, but have incompletely characterized neuronal functions. We show that short hairpin RNA-mediated knockdown of erbin and knockdown or genetic ablation of delta-catenin severely impaired dendritic morphogenesis in hippocampal neurons. Simultaneous loss of erbin and delta-catenin does not enhance severity of this phenotype. The dendritic phenotype observed after erbin depletion is rescued by overexpression of delta-catenin and requires a domain in delta-catenin that has been shown to regulate dendritic branching. Knockdown of delta-catenin cannot be rescued by overexpression of erbin, indicating that erbin is upstream of delta-catenin. delta-Catenin-null neurons have no alterations in global levels of active Rac1/RhoA. Knockdown of erbin results in alterations in localization of delta-catenin. These results suggest a critical role for erbin in regulating dendritic morphogenesis by maintaining appropriate localization of delta-catenin.

The best disease-linked Cl- channel hBest1 regulates Ca V 1 (L-type) Ca2+ channels via src-homology-binding domains

Mutations in the bestrophin-1 (Best1) gene are linked to several kinds of macular degeneration in both humans and dogs. Although bestrophins have been shown clearly to be Cl(-) ion channels, it is controversial whether Cl(-) channel dysfunction can explain the diseases. It has been suggested that bestrophins are multifunctional proteins: they may regulate voltage-gated Ca(2+) channels in addition to functioning as Cl(-) channels. Here, we show that human Best1 gene (hBest1) differentially modulates Ca(V)1.3 (L-type) voltage-gated Ca(2+) channels through association with the Ca(V)beta subunit. In transfected human embryonic kidney 293 cells, hBest1 inhibited Ca(V)1.3. Inhibition of Ca(V)1.3 was not observed in the absence of the beta subunit. Also, the hBest1 C terminus binds to Ca(V)beta subunits, suggesting that the effect of hBest1 was mediated by the Ca(V)beta subunit. The region of hBest1 responsible for the effect was localized to a region (amino acids 330-370) in the cytoplasmic C terminus that contains a predicted src-homology-binding domain that is not present in other bestrophin subtypes. Mutation of Pro(330) and Pro(334) abolished the effects of hBest1 on Ca(V)1.3. The effect was specific to hBest1; it was not observed with mouse Best1 (mBest1), mBest2, or mBest3. Wild-type hBest1 and the disease-causing mutants R92S, G299R, and D312N inhibited Ca(V) currents the same amount, whereas the A146K and G222E mutants were less effective. We propose that hBest1 regulates Ca(V) channels by interacting with the Ca(V)beta subunit and altering channel availability. Our findings reveal a novel function of bestrophin in regulation of Ca(V) channels and suggest a possible mechanism for the role of hBest1 in macular degeneration.

Modulation of voltage- and Ca2+-dependent gating of CaV1.3 L-type calcium channels by alternative splicing of a C-terminal regulatory domain

Low voltage activation of Ca(V)1.3 L-type Ca(2+) channels controls excitability in sensory cells and central neurons as well as sinoatrial node pacemaking. Ca(V)1.3-mediated pacemaking determines neuronal vulnerability of dopaminergic striatal neurons affected in Parkinson disease. We have previously found that in Ca(V)1.4 L-type Ca(2+) channels, activation, voltage, and calcium-dependent inactivation are controlled by an intrinsic distal C-terminal modulator. Because alternative splicing in the Ca(V)1.3 alpha1 subunit C terminus gives rise to a long (Ca(V)1.3(42)) and a short form (Ca(V)1.3(42A)), we investigated if a C-terminal modulatory mechanism also controls Ca(V)1.3 gating. The biophysical properties of both splice variants were compared after heterologous expression together with beta3 and alpha2delta1 subunits in HEK-293 cells. Activation of calcium current through Ca(V)1.3(42A) channels was more pronounced at negative voltages, and inactivation was faster because of enhanced calcium-dependent inactivation. By investigating several Ca(V)1.3 channel truncations, we restricted the modulator activity to the last 116 amino acids of the C terminus. The resulting Ca(V)1.3(DeltaC116) channels showed gating properties similar to Ca(V)1.3(42A) that were reverted by co-expression of the corresponding C-terminal peptide C(116). Fluorescence resonance energy transfer experiments confirmed an intramolecular protein interaction in the C terminus of Ca(V)1.3 channels that also modulates calmodulin binding. These experiments revealed a novel mechanism of channel modulation enabling cells to tightly control Ca(V)1.3 channel activity by alternative splicing. The absence of the C-terminal modulator in short splice forms facilitates Ca(V)1.3 channel activation at lower voltages expected to favor Ca(V)1.3 activity at threshold voltages as required for modulation of neuronal firing behavior and sinoatrial node pacemaking.

Ultrastructural evidence for pre- and postsynaptic localization of Cav1.2 L-type Ca2+ channels in the rat hippocampus

In the hippocampal formation, Ca(v)1.2 (L-type) voltage-gated Ca(2+) channels mediate Ca(2+) signals that can trigger long-term alterations in synaptic efficacy underlying learning and memory. Immunocytochemical studies indicate that Ca(v)1.2 channels are localized mainly in the soma and proximal dendrites of hippocampal pyramidal neurons, but electrophysiological data suggest a broader distribution of these channels. To define the subcellular substrates underlying Ca(v)1.2 Ca(2+) signals, we analyzed the localization of Ca(v)1.2 in the hippocampal formation by using antibodies against the pore-forming alpha(1)-subunit of Ca(v)1.2 (alpha(1)1.2). By light microscopy, alpha(1)1.2-like immunoreactivity (alpha(1)1.2-IR) was detected in pyramidal cell soma and dendritic fields of areas CA1-CA3 and in granule cell soma and fibers in the dentate gyrus. At the electron microscopic level, alpha(1)1.2-IR was localized in dendrites, but also in axons, axon terminals, and glial processes in all hippocampal subfields. Plasmalemmal immunogold particles representing alpha(1)1.2-IR were more significant for small- than large-caliber dendrites and were largely associated with extrasynaptic regions in dendritic spines and axon terminals. These findings provide the first detailed ultrastructural analysis of Ca(v)1.2 localization in the brain and support functionally diverse roles of these channels in the hippocampal formation.

Developmentally regulated alternative splicing of densin modulates protein-protein interaction and subcellular localization

Densin is a member of the leucine-rich repeat (LRR) and PDZ domain (LAP) protein family that binds several signaling molecules via its C-terminal domains, including calcium/calmodulin-dependent protein kinase II (CaMKII). In this study, we identify several novel mRNA splice variants of densin that are differentially expressed during development. The novel variants share the LRR domain but are either prematurely truncated or contain internal deletions relative to mature variants of the protein (180 kDa), thus removing key protein-protein interaction domains. For example, CaMKIIalpha coimmunoprecipitates with densin splice variants containing an intact C-terminal domain from lysates of transfected HEK293 cells, but not with variants that only contain N-terminal domains. Immunoblot analyses using antibodies to peptide epitopes in the N- and C- terminal domains of densin are consistent with developmental regulation of splice variant expression in brain. Moreover, putative splice variants display different subcellular fractionation patterns in brain extracts. Expression of green fluorescent protein (GFP)-fused densin splice variants in HEK293 cells shows that the LRR domain can target densin to a plasma membrane-associated compartment, but that the splice variants are differentially localized and have potentially distinct effects on cell morphology. In combination, these data show that densin splice variants have distinct functional characteristics suggesting multiple roles during neuronal development.

Ca2+-binding proteins tune Ca2+-feedback to Cav1.3 channels in mouse auditory hair cells

Sound coding at the auditory inner hair cell synapse requires graded changes in neurotransmitter release, triggered by sustained activation of presynaptic Ca(v)1.3 voltage-gated Ca(2+) channels. Central to their role in this regard, Ca(v)1.3 channels in inner hair cells show little Ca(2+)-dependent inactivation, a fast negative feedback regulation by incoming Ca(2+) ions, which depends on calmodulin association with the Ca(2+) channel alpha(1) subunit. Ca(2+)-dependent inactivation characterizes nearly all voltage-gated Ca(2+) channels including Ca(v)1.3 in other excitable cells. The mechanism underlying the limited autoregulation of Ca(v)1.3 in inner hair cells remains a mystery. Previously, we established calmodulin-like Ca(2+)-binding proteins in the brain and retina (CaBPs) as essential modulators of voltage-gated Ca(2+) channels. Here, we demonstrate that CaBPs differentially modify Ca(2+) feedback to Ca(v)1.3 channels in transfected cells and explore their significance for Ca(v)1.3 regulation in inner hair cells. Of multiple CaBPs detected in inner hair cells (CaBP1, CaBP2, CaBP4 and CaBP5), CaBP1 most efficiently blunts Ca(2+)-dependent inactivation of Ca(v)1.3. CaBP1 and CaBP4 both interact with calmodulin-binding sequences in Ca(v)1.3, but CaBP4 more weakly inhibits Ca(2+)-dependent inactivation than CaBP1. Ca(2+)-dependent inactivation is marginally greater in inner hair cells from CaBP4(-/-) than from wild-type mice, yet CaBP4(-/-) mice are not hearing-impaired. In contrast to CaBP4, CaBP1 is strongly localized at the presynaptic ribbon synapse of adult inner hair cells both in wild-type and CaBP4(-/-) mice and therefore is positioned to modulate native Ca(v)1.3 channels. Our results reveal unexpected diversity in the strengths of CaBPs as Ca(2+) channel modulators, and implicate CaBP1 rather than CaBP4 in conferring the anomalous slow inactivation of Ca(v)1.3 Ca(2+) currents required for auditory transmission.