Congenital stationary night blindness type 2 mutations S229P, G369D, L1068P, and W1440X alter channel gating or functional expression of Ca(v)1.4 L-type Ca2+ channels

A non-conducting role of the Cav1.4 Ca2+ channel drives homeostatic plasticity at the cone photoreceptor synapse

In congenital stationary night blindness, type 2 (CSNB2)-a disorder involving the Cav1.4 (L-type) Ca2+ channel-visual impairment is mild considering that Cav1.4 mediates synaptic release from rod and cone photoreceptors. Here, we addressed this conundrum using a Cav1.4 knockout (KO) mouse and a knock-in (G369i KI) mouse expressing a non-conducting Cav1.4. Surprisingly, Cav3 (T-type) Ca2+ currents were detected in cones of G369i KI mice and Cav1.4 KO mice but not in cones of wild-type mouse, ground squirrels, and macaque retina. Whereas Cav1.4 KO mice are blind, G369i KI mice exhibit normal photopic (i.e. cone-mediated) visual behavior. Cone synapses, which fail to form in Cav1.4 KO mice, are present, albeit enlarged, and with some errors in postsynaptic wiring in G369i KI mice. While Cav1.4 KO mice lack evidence of cone synaptic responses, electrophysiological recordings in G369i KI mice revealed nominal transmission from cones to horizontal cells and bipolar cells. In CSNB2, we propose that Cav3 channels maintain cone synaptic output provided that the nonconducting role of Cav1.4 in cone synaptogenesis remains intact. Our findings reveal an unexpected form of homeostatic plasticity that relies on a non-canonical role of an ion channel.

A non-conducting role of the Cav1.4 Ca2+ channel drives homeostatic plasticity at the cone photoreceptor synapse

In congenital stationary night blindness type 2 (CSNB2)-a disorder involving the Cav1.4 (L-type) Ca2+ channel-visual impairment is mild considering that Cav1.4 mediates synaptic release from rod and cone photoreceptors. Here, we addressed this conundrum using a Cav1.4 knockout (KO) mouse and a knock-in (G369i KI) mouse expressing a non-conducting Cav1.4. Surprisingly, Cav3 (T-type) Ca2+ currents were detected in cones of G369i KI mice and Cav1.4 KO mice but not in cones of wild-type mouse, ground squirrel, and macaque retina. Whereas Cav1.4 KO mice are blind, G369i KI mice exhibit normal photopic (i.e., cone-mediated) visual behavior. Cone synapses, which fail to form in Cav1.4 KO mice, are present, albeit enlarged, and with some errors in postsynaptic wiring in G369i KI mice. While Cav1.4 KO mice lack evidence of cone synaptic responses, electrophysiological recordings in G369i KI mice revealed nominal transmission from cones to horizontal cells and bipolar cells. In CSNB2, we propose that Cav3 channels maintain cone synaptic output provided that the nonconducting role of Cav1.4 in cone synaptogenesis remains intact. Our findings reveal an unexpected form of homeostatic plasticity that relies on a non-canonical role of an ion channel.

Characterization of two pathological gating-charge substitutions in Cav1.4 L-type calcium channels

Cav1.4 L-type calcium channels are predominantly expressed at the photoreceptor terminals and in bipolar cells, mediating neurotransmitter release. Mutations in its gene, CACNA1F, can cause congenital stationary night-blindness type 2 (CSNB2). Due to phenotypic variability in CSNB2, characterization of pathological variants is necessary to better determine pathological mechanism at the site of action. A set of known mutations affects conserved gating charges in the S4 voltage sensor, two of which have been found in male CSNB2 patients. Here, we describe two disease-causing Cav1.4 mutations with gating charge neutralization, exchanging an arginine 964 with glycine (RG) or arginine 1288 with leucine (RL). In both, charge neutralization was associated with a reduction channel expression also reflected in smaller ON gating currents. In RL channels, the strong decrease in whole-cell current densities might additionally be explained by a reduction of single-channel currents. We further identified alterations in their biophysical properties, such as a hyperpolarizing shift of the activation threshold and an increase in slope factor of activation and inactivation. Molecular dynamic simulations in RL substituted channels indicated water wires in both, resting and active, channel states, suggesting the development of omega (ω)currents as a new pathological mechanism in CSNB2. This sum of the respective channel property alterations might add to the differential symptoms in patients beside other factors, such as genomic and environmental deviations.

Ribbon Synapses and Retinal Disease: Review

Synaptic ribbons are presynaptic protein complexes that are believed to be important for the transmission of sensory information in the visual system. Ribbons are selectively associated with those synapses where graded changes in membrane potential drive continuous neurotransmitter release. Defective synaptic transmission can arise as a result of the mutagenesis of a single ribbon component. Visual diseases that stem from malfunctions in the presynaptic molecular machinery of ribbon synapses in the retina are rare. In this review, we provide an overview of synaptopathies that give rise to retinal malfunction and our present understanding of the mechanisms that underlie their pathogenesis and discuss muscular dystrophies that exhibit ribbon synapse involvement in the pathology.

Cav1.4 congenital stationary night blindness is associated with an increased rate of proteasomal degradation

Pathogenic, generally loss-of-function, variants in CACNA1F, encoding the Ca_v1.4α₁ calcium channel, underlie congenital stationary night blindness type 2 (CSNB2), a rare inherited retinal disorder associated with visual disability. To establish the underlying pathomechanism, we investigated 10 clinically derived CACNA1F missense variants located across pore-forming domains, connecting loops, and the carboxy-tail domain of the Ca_v1.4α subunit. Homology modeling showed that all variants cause steric clashes; informatics analysis correctly predicted pathogenicity for 7/10 variants. In vitro analyses demonstrated that all variants cause a decrease in current, global expression, and protein stability and act through a loss-of-function mechanism and suggested that the mutant Ca_v1.4α proteins were degraded by the proteasome. We showed that the reduced current for these variants could be significantly increased through treatment with clinical proteasome inhibitors. In addition to facilitating clinical interpretation, these studies suggest that proteasomal inhibition represents an avenue of potential therapeutic intervention for CSNB2.

Calcium Channels in Retinal Function and Disease

Voltage-gated Ca2+ (Cav) channels play pivotal roles in regulating gene transcription, neuronal excitability, and neurotransmitter release. To meet the spatial and temporal demands of visual signaling, Cav channels exhibit unusual properties in the retina compared to their counterparts in other areas of the nervous system. In this article, we review current concepts regarding the specific subtypes of Cav channels expressed in the retina, their intrinsic properties and forms of modulation, and how their dysregulation could lead to retinal disease.

Two novel CACNA1F gene mutations cause two different phenotypes: Aland Eye Disease and incomplete Congenital Stationary Night Blindness

Congenital Stationary Night Blindness type 2 (CSNB2) and Aland island Eye Disease (AIED) associated with CACNA1F mutation demonstrate a significant phenotype overlapping. We report two cases with different clinical presentation carrying two novel mutations in CACNA1F gene. Subjects underwent a complete neurophtahlmological examination associated with structural and electrofunctional insight. Next Generation Sequencing (NGS) analysis of 31 genes previously associated with retinal dystrophy (RD) was performed. Messenger RNAs derived from probands 'peripheral blood samples were analyzed by RT-PCR and cDNA sequencing. The neuro-ophthalmological examinations revealed different clinical, structural and morphological presentations, more severe in patient 1 compared with patient 2. Molecular analysis revealed, that both patients had the hemizygous form of two novel mutations in CACNA1F gene. Patient 1 presented a duplication (c.425dupC) in exon 4, resulting in shifting of the reading frame with the insertion of a premature Stop codon. In Patient 2 variant c.5156G > C localized in the donor's splicing site of exon 43 was identified. Complementary DNA sequencing demonstrated skipping of exon 43 with a deletion of 55 amino acids that causes a frame shift with insertion of a Stop codon. These findings suggest that the effect and the localization of the mutations in the CACNA1F gene can explain different clinical phenotypes. Clinical spectrum is more severe and resembles the AIED phenotype when the mutation affects the first part of the protein, while it is more similar to CSNB2 if the mutation is localized at the end of the protein. Genetic testing results to be an essential tool to provide more accurate diagnosis and prognosis in patients with inherited retinal degenerative disorders and could help, in the future, to develop more specific therapeutic strategies.

Congenital stationary night blindness: an update and review of the disease spectrum in Saudi Arabia

Congenital stationary night blindness (CSNB) is a group of rare, mainly stationary disorders of the retina, resulting from dysfunction of several specific and essential visual processing mechanisms. The inheritance is often recessive and as such, CSNB may be more common among populations with a high degree of consanguinity. Here, we present a topic update and a review of the clinical and molecular genetic spectrum of CSNB in Saudi Arabia. Since a major review article on CSNB in 2015, which described 17 genes underlying CSNB, an additional four genes have been incriminated in autosomal recessive CSNB: RIMS2, GNB3, GUCY2D and ABCA4. These have been associated with syndromic cone-rod synaptic disease, ON bipolar cell dysfunction with reduced cone sensitivity, CSNB with dysfunction of the phototransduction (Riggs type) and CSNB with cone-rod dystrophy, respectively. In Saudi Arabia, a total of 24 patients with CSNB were identified, using a combination of literature search and retrospective study of previously unpublished cases. Recessive mutations in TRPM1 and CABP4 accounted for the majority of cases (5 and 13 for each gene, respectively). These genes were associated with complete (cCSNB) and incomplete (icCSNB), respectively, and were associated with high myopia in the former and hyperopia in the latter. Four novel mutations were identified. For the first time, we describe the fundus albipunctatus in two patients from Saudi Arabia, caused by recessive mutation in RDH5 and RPE65, where the former in addition featured findings compatible with cone dystrophy. No cases were identified with any dominantly inherited CSNB.

Cav1.4 dysfunction and congenital stationary night blindness type 2

Cav1.4 L-type Ca2+ channels are predominantly expressed in retinal neurons, particularly at the photoreceptor terminals where they mediate sustained Ca2+ entry needed for continuous neurotransmitter release at their ribbon synapses. Cav1.4 channel gating properties are controlled by accessory subunits, associated regulatory proteins, and also alternative splicing. In humans, mutations in the CACNA1F gene encoding for Cav1.4 channels are associated with X-linked retinal disorders such as congenital stationary night blindness type 2. Mutations in the Cav1.4 protein result in a spectrum of altered functional channel activity. Several mouse models broadened our understanding of the role of Cav1.4 channels not only as Ca2+ source at retinal synapses but also as synaptic organizers. In this review, we highlight different structural and functional phenotypes of Cav1.4 mutations that might also occur in patients with congenital stationary night blindness type 2. A further important yet mostly neglected aspect that we discuss is the influence of alternative splicing on channel dysfunction. We conclude that currently available functional phenotyping strategies should be refined and summarize potential specific therapeutic options for patients carrying Cav1.4 mutations. Importantly, the development of new therapeutic approaches will permit a deeper understanding of not only the disease pathophysiology but also the physiological function of Cav1.4 channels in the retina.

Voltage-Gated Ca2+-Channel α1-Subunit de novo Missense Mutations: Gain or Loss of Function - Implications for Potential Therapies

This review summarizes our current knowledge of human disease-relevant genetic variants within the family of voltage gated Ca2+ channels. Ca2+ channelopathies cover a wide spectrum of diseases including epilepsies, autism spectrum disorders, intellectual disabilities, developmental delay, cerebellar ataxias and degeneration, severe cardiac arrhythmias, sudden cardiac death, eye disease and endocrine disorders such as congential hyperinsulinism and hyperaldosteronism. A special focus will be on the rapidly increasing number of de novo missense mutations identified in the pore-forming α1-subunits with next generation sequencing studies of well-defined patient cohorts. In contrast to likely gene disrupting mutations these can not only cause a channel loss-of-function but can also induce typical functional changes permitting enhanced channel activity and Ca2+ signaling. Such gain-of-function mutations could represent therapeutic targets for mutation-specific therapy of Ca2+-channelopathies with existing or novel Ca2+-channel inhibitors. Moreover, many pathogenic mutations affect positive charges in the voltage sensors with the potential to form gating-pore currents through voltage sensors. If confirmed in functional studies, specific blockers of gating-pore currents could also be of therapeutic interest.

A Review of Genetic and Physiological Disease Mechanisms Associated With Cav1 Channels: Implications for Incomplete Congenital Stationary Night Blindness Treatment

Calcium channels are crucial to a number of cellular functions. The high voltage-gated calcium channel family comprise four heteromeric channels (Cav1.1-1.4) that function in a similar manner, but that have distinct expression profiles. Three of the pore-forming α₁ subunits are located on autosomes and the forth on the X chromosome, which has consequences for the type of pathogenic mutation and the disease mechanism associated with each gene. Mutations in this family of channels are associated with malignant hyperthermia (Cav1.1), various QT syndromes (Cav1.2), deafness (Cav1.3), and incomplete congenital stationary night blindness (iCSNB; Cav1.4). In this study we performed a bioinformatic analysis on reported mutations in all four Cav α₁ subunits and correlated these with variant frequency in the general population, phenotype and the effect on channel conductance to produce a comprehensive composite Cav1 mutation analysis. We describe regions of mutation clustering, identify conserved residues that are mutated in multiple family members and regions likely to cause a loss- or gain-of-function in Cav1.4. Our research highlights that therapeutic treatments for each of the Cav1 channels will have to consider channel-specific mechanisms, especially for the treatment of X-linked iCSNB.

The molecular basis of the inhibition of CaV1 calcium-dependent inactivation by the distal carboxy tail

Ca2+/calmodulin-dependent inactivation (CDI) of CaV channels is a critical regulatory process that tunes the kinetics of Ca2+ entry for different cell types and physiologic responses. CDI is mediated by calmodulin (CaM), which is bound to the IQ domain of the CaV carboxy tail. This modulatory process is tailored by alternative splicing such that select splice variants of CaV1.3 and CaV1.4 contain a long distal carboxy tail (DCT). The DCT harbors an inhibitor of CDI (ICDI) module that competitively displaces CaM from the IQ domain, thereby diminishing CDI. While this overall mechanism is now well described, the detailed interactions required for ICDI binding to the IQ domain are yet to be elucidated. Here, we perform alanine-scanning mutagenesis of the IQ and ICDI domains and evaluate the contribution of neighboring regions to CDI inhibition. Through FRET binding analysis, we identify functionally relevant residues within the CaV1.3 IQ domain and the CaV1.4 ICDI and nearby A region, which are required for high-affinity IQ/ICDI binding. Importantly, patch-clamp recordings demonstrate that disruption of this interaction commensurately diminishes ICDI function resulting in the re-emergence of CDI in mutant channels. Furthermore, CaV1.2 channels harbor a homologous DCT; however, the ICDI region of this channel does not appear to appreciably modulate CaV1.2 CDI. Yet coexpression of CaV1.2 ICDI with select CaV1.3 splice variants significantly disrupts CDI, implicating a cross-channel modulatory scheme in cells expressing both channel subtypes. In all, these findings provide new insights into a molecular rheostat that fine-tunes Ca2+-entry and supports normal neuronal and cardiac function.

Functional impact of a congenital stationary night blindness type 2 mutation depends on subunit composition of Cav1.4 Ca2+ channels

Voltage-gated Ca_v1 and Ca_v2 Ca²⁺ channels are comprised of a pore-forming α₁ subunit (Ca_v1.1-1.4, Ca_v2.1-2.3) and auxiliary β (β_1-4) and α₂δ (α₂δ-1-4) subunits. The properties of these channels vary with distinct combinations of Ca_v subunits and alternative splicing of the encoding transcripts. Therefore, the impact of disease-causing mutations affecting these channels may depend on the identities of Ca_v subunits and splice variants. Here, we analyzed the effects of a congenital stationary night blindness type 2 (CSNB2)-causing mutation, I745T (IT), in Ca_v1.4 channels typical of those in human retina: Ca_v1.4 splice variants with or without exon 47 (Ca_v1.4+ex47 and Ca_v1.4Δex47, respectively), and the auxiliary subunits, β_2X13 and α₂δ-4. We find that IT caused both Ca_v1.4 splice variants to activate at significantly more negative voltages and with slower deactivation kinetics than the corresponding WT channels. These effects of the IT mutation, along with unexpected alterations in ion selectivity, were generally larger in channels lacking exon 47. The weaker ion selectivity caused by IT led to hyperpolarizing shifts in the reversal potential and large outward currents that were evident in channels containing the auxiliary subunits β_2X13 and α₂δ-4 but not in those with β_2A and α₂δ-1. We conclude that the IT mutation stabilizes channel opening and alters ion selectivity of Ca_v1.4 in a manner that is strengthened by exclusion of exon 47 and inclusion of β_2X13 and α₂δ-4. Our results reveal complex actions of IT in modifying the properties of Ca_v1.4 channels, which may influence the pathological consequences of this mutation in retinal photoreceptors.

Channelopathies of voltage-gated L-type Cav1.3/α1D and T-type Cav3.1/α1G Ca2+ channels in dysfunction of heart automaticity

The heart automaticity is a fundamental physiological function in vertebrates. The cardiac impulse is generated in the sinus node by a specialized population of spontaneously active myocytes known as "pacemaker cells." Failure in generating or conducting spontaneous activity induces dysfunction in cardiac automaticity. Several families of ion channels are involved in the generation and regulation of the heart automaticity. Among those, voltage-gated L-type Cav1.3 (α1D) and T-type Cav3.1 (α1G) Ca²⁺ channels play important roles in the spontaneous activity of pacemaker cells. Ca²⁺ channel channelopathies specifically affecting cardiac automaticity are considered rare. Recent research on familial disease has identified mutations in the Cav1.3-encoding CACNA1D gene that underlie congenital sinus node dysfunction and deafness (OMIM # 614896). In addition, both Cav1.3 and Cav3.1 channels have been identified as pathophysiological targets of sinus node dysfunction and heart block, caused by congenital autoimmune disease of the cardiac conduction system. The discovery of channelopathies linked to Cav1.3 and Cav3.1 channels underscores the importance of Ca²⁺ channels in the generation and regulation of heart's automaticity.

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.

Angiotensin-Receptor-Associated Protein Modulates Ca2+ Signals in Photoreceptor and Mossy Fiber cells

Fast, precise and sustained neurotransmission requires graded Ca²⁺ signals at the presynaptic terminal. Neurotransmitter release depends on a complex interplay of Ca²⁺ fluxes and Ca²⁺ buffering in the presynaptic terminal that is not fully understood. Here, we show that the angiotensin-receptor-associated protein (ATRAP) localizes to synaptic terminals throughout the central nervous system. In the retinal photoreceptor synapse and the cerebellar mossy fiber-granule cell synapse, we find that ATRAP is involved in the generation of depolarization-evoked synaptic Ca²⁺ transients. Compared to wild type, Ca²⁺ imaging in acutely isolated preparations of the retina and the cerebellum from ATRAP knockout mice reveals a significant reduction of the sarcoendoplasmic reticulum (SR) Ca²⁺-ATPase (SERCA) activity. Thus, in addition to its conventional role in angiotensin signaling, ATRAP also modulates presynaptic Ca²⁺ signaling within the central nervous system.

Disinhibition of intrinsic photosensitive retinal ganglion cells in patients with X-linked congenital stationary night blindness

PURPOSE

To assess the pupil light response (PLR) to chromatic stimulation in patients with different types of X-linked congenital stationary night blindness (CSNB).

METHODS

Eight patients with CSNB due to CACNA1F and NYX mutations were exposed to blue and red light stimuli, and PLR was evaluated using infrared video pupillography. Pupil responses were compared between CSNB patients and healthy subjects (n = 34) at baseline, at maximum of constriction, for post-illumination pupil responses (PIPR) and the slope of redilation using Cohen's d. A subgroup comparison was performed descriptively between CACNA1F and NYX associated CSNB patients using the same parameters.

RESULTS

In CSNB, smaller baseline pupil diameters compared to healthy subjects were measured both before blue and red light stimulation (d = 1.44-1.625). The maximum constriction to blue light stimuli was smaller for the CSNB group compared to healthy subjects (d = 1.251) but not for red light stimuli (d = 0.449). Pupil response latencies were prolonged in CSNB for both light stimuli (d = -1.53 for blue and d = -1.011 for red stimulation). No relevant differences were found between the CSNB group and healthy subjects for PIPR (d = 0.01), but the slope of redilation was smaller for CSNB patients (d = 2.12). Paradoxical pupil constriction at light offset was not seen in our patients.

CONCLUSION

A reduced redilation and smaller baseline pupil diameters for patients with CSNB indicate a disinhibition of intrinsically photosensitive retinal ganglion cells due to affected post-photoreceptor transduction via bipolar cells and can explain the pupillary behavior in our patient group.

Voltage-Gated Calcium Channels: Key Players in Sensory Coding in the Retina and the Inner Ear

Calcium influx through voltage-gated Ca (Ca_V) channels is the first step in synaptic transmission. This review concerns Ca_V channels at ribbon synapses in primary sense organs and their specialization for efficient coding of stimuli in the physical environment. Specifically, we describe molecular, biochemical, and biophysical properties of the Ca_V channels in sensory receptor cells of the retina, cochlea, and vestibular apparatus, and we consider how such properties might change over the course of development and contribute to synaptic plasticity. We pay particular attention to factors affecting the spatial arrangement of Ca_V channels at presynaptic, ribbon-type active zones, because the spatial relationship between Ca_V channels and release sites has been shown to affect synapse function critically in a number of systems. Finally, we review identified synaptopathies affecting sensory systems and arising from dysfunction of L-type, Ca_V1.3, and Ca_V1.4 channels or their protein modulatory elements.

Ca2+ Channels in Anterior Pituitary Somatotrophs: A Therapeutic Perspective

Ca2+ influx through voltage-gated Ca2+ channels (VGCCs) plays a key role in GH secretion. In this review, we summarize the current state of knowledge regarding the physiology and molecular machinery of VGCCs in pituitary somatotrophs. We next discuss the possible involvement of Ca2+ channelopathies in pituitary disease and the potential use of Ca2+ channel blockers to treat pituitary disease. Various types of VGCCs exist in pituitary cells. However, because L-type Ca2+ channels (LTCCs) contribute the major component to Ca2+ influx in somatotrophs, lactotrophs, and corticotrophs, we focused on these channels. An increasing number of studies in recent years have linked genetic missense mutations in LTCCs to diseases of the human cardiovascular, nervous, and endocrine systems. These disease-associated genetic mutations occur at homologous functional positions (activation gates) in LTCCs. Thus, it is plausible that similar homologous missense mutations in pituitary LTCCs can cause abnormal hormone secretion and underlying pituitary disorders. The existence of LTCCs in pituitary cells opens questions about their sensitivity to dihydropyridines, a group of selective LTCC blockers. The dihydropyridine sensitivity of pituitary cells, as with any other excitable cell, depends primarily on two parameters: the pattern of their electrical activity and the dihydropyridine sensitivity of their LTCC isoforms. These two parameters are discussed in detail in relation to somatotrophs. These discussions are also relevant to lactotrophs and corticotrophs. High dihydropyridine sensitivity may facilitate their use as drugs to treat pituitary oversecretion disorders such as acromegaly, hyperprolactinemia, and Cushing disease.

Calcium channel gating

Tuned calcium entry through voltage-gated calcium channels is a key requirement for many cellular functions. This is ensured by channel gates which open during membrane depolarizations and seal the pore at rest. The gating process is determined by distinct sub-processes: movement of voltage-sensing domains (charged S4 segments) as well as opening and closure of S6 gates. Neutralization of S4 charges revealed that pore opening of CaV1.2 is triggered by a "gate releasing" movement of all four S4 segments with activation of IS4 (and IIIS4) being a rate-limiting stage. Segment IS4 additionally plays a crucial role in channel inactivation. Remarkably, S4 segments carrying only a single charged residue efficiently participate in gating. However, the complete set of S4 charges is required for stabilization of the open state. Voltage clamp fluorometry, the cryo-EM structure of a mammalian calcium channel, biophysical and pharmacological studies, and mathematical simulations have all contributed to a novel interpretation of the role of voltage sensors in channel opening, closure, and inactivation. We illustrate the role of the different methodologies in gating studies and discuss the key molecular events leading CaV channels to open and to close.

Channeling Vision: CaV1.4-A Critical Link in Retinal Signal Transmission

Voltage-gated calcium channels (VGCC) are key to many biological functions. Entry of Ca2+ into cells is essential for initiating or modulating important processes such as secretion, cell motility, and gene transcription. In the retina and other neural tissues, one of the major roles of Ca2+-entry is to stimulate or regulate exocytosis of synaptic vesicles, without which synaptic transmission is impaired. This review will address the special properties of one L-type VGCC, CaV1.4, with particular emphasis on its role in transmission of visual signals from rod and cone photoreceptors (hereafter called "photoreceptors," to the exclusion of intrinsically photoreceptive retinal ganglion cells) to the second-order retinal neurons, and the pathological effects of mutations in the CACNA1F gene which codes for the pore-forming α1F subunit of CaV1.4.

Clinical Characteristics, Mutation Spectrum, and Prevalence of Åland Eye Disease/Incomplete Congenital Stationary Night Blindness in Denmark

Purpose

To assess clinical characteristics, foveal structure, mutation spectrum, and prevalence rate of Åland eye disease (AED)/incomplete congenital stationary night blindness (iCSNB).

Methods

A retrospective survey included individuals diagnosed with AED at a national low-vision center from 1980 to 2014. A subset of affected males underwent ophthalmologic examinations including psychophysical tests, full-field electroretinography, and spectral-domain optical coherence tomography.

Results

Over the 34-year period, 74 individuals from 35 families were diagnosed with AED. Sixty individuals from 29 families participated in a follow-up study of whom 59 harbored a CACNA1F mutation and 1 harbored a CABP4 mutation. Among the subjects with a CACNA1F mutation, subnormal visual acuity was present in all, nystagmus was present in 63%, and foveal hypoplasia was observed in 25/43 subjects. Foveal pit volume was significantly reduced as compared to normal (P < 0.0001). Additionally, outer segment length at the fovea was measured in 46 subjects and found to be significantly reduced as compared to normal (P < 0.001). Twenty-nine CACNA1F variations were detected among 34 families in the total cohort, and a novel CABP4 variation was identified in one family. The estimated mean birth prevalence rate was 1 per 22,000 live-born males.

Conclusions

Our data support the viewpoint that AED, iCSNB, and X-linked cone–rod dystrophy 3 are designations that refer to a broad, continuous spectrum of clinical appearances caused in the majority by a variety of mutations in CACNA1F. We argue that the original designation AED should be used for this entity.

An autism-associated mutation in CaV1.3 channels has opposing effects on voltage- and Ca(2+)-dependent regulation

CaV1.3 channels are a major class of L-type Ca(2+) channels which contribute to the rhythmicity of the heart and brain. In the brain, these channels are vital for excitation-transcription coupling, synaptic plasticity, and neuronal firing. Moreover, disruption of CaV1.3 function has been associated with several neurological disorders. Here, we focus on the de novo missense mutation A760G which has been linked to autism spectrum disorder (ASD). To explore the role of this mutation in ASD pathogenesis, we examined the effects of A760G on CaV1.3 channel gating and regulation. Introduction of the mutation severely diminished the Ca(2+)-dependent inactivation (CDI) of CaV1.3 channels, an important feedback system required for Ca(2+) homeostasis. This reduction in CDI was observed in two major channel splice variants, though to different extents. Using an allosteric model of channel gating, we found that the underlying mechanism of CDI reduction is likely due to enhanced channel opening within the Ca(2+)-inactivated mode. Remarkably, the A760G mutation also caused an opposite increase in voltage-dependent inactivation (VDI), resulting in a multifaceted mechanism underlying ASD. When combined, these regulatory deficits appear to increase the intracellular Ca(2+) concentration, thus potentially disrupting neuronal development and synapse formation, ultimately leading to ASD.

Arrhythmogenesis in Timothy Syndrome is associated with defects in Ca(2+)-dependent inactivation

Timothy Syndrome (TS) is a multisystem disorder, prominently featuring cardiac action potential prolongation with paroxysms of life-threatening arrhythmias. The underlying defect is a single de novo missense mutation in Ca_V1.2 channels, either G406R or G402S. Notably, these mutations are often viewed as equivalent, as they produce comparable defects in voltage-dependent inactivation and cause similar manifestations in patients. Yet, their effects on calcium-dependent inactivation (CDI) have remained uncertain. Here, we find a significant defect in CDI in TS channels, and uncover a remarkable divergence in the underlying mechanism for G406R versus G402S variants. Moreover, expression of these TS channels in cultured adult guinea pig myocytes, combined with a quantitative ventricular myocyte model, reveals a threshold behaviour in the induction of arrhythmias due to TS channel expression, suggesting an important therapeutic principle: a small shift in the complement of mutant versus wild-type channels may confer significant clinical improvement.

Timothy Syndrome (TS) is a multisystem disorder caused by two mutations leading to dysfunction of the Ca_V1.2 channel. Here, Dick et al. uncover a major and mechanistically divergent effect of both mutations on Ca²⁺/calmodulin-dependent inactivation of Ca_V1.2 channels, suggesting genetic variant-tailored therapy for TS treatment.

Ion Channels in the Eye: Involvement in Ocular Pathologies

The eye is the sensory organ of vision. There, the retina transforms photons into electrical signals that are sent to higher brain areas to produce visual sensations. In the light path to the retina, different types of cells and tissues are involved in maintaining the transparency of avascular structures like the cornea or lens, while others, like the retinal pigment epithelium, have a critical role in the maintenance of photoreceptor function by regenerating the visual pigment. Here, we have reviewed the roles of different ion channels expressed in ocular tissues (cornea, conjunctiva and neurons innervating the ocular surface, lens, retina, retinal pigment epithelium, and the inflow and outflow systems of the aqueous humor) that are involved in ocular disease pathophysiologies and those whose deletion or pharmacological modulation leads to specific diseases of the eye. These include pathologies such as retinitis pigmentosa, macular degeneration, achromatopsia, glaucoma, cataracts, dry eye, or keratoconjunctivitis among others. Several disease-associated ion channels are potential targets for pharmacological intervention or other therapeutic approaches, thus highlighting the importance of these channels in ocular physiology and pathophysiology.

RIM1/2-Mediated Facilitation of Cav1.4 Channel Opening Is Required for Ca2+-Stimulated Release in Mouse Rod Photoreceptors

Night blindness can result from impaired photoreceptor function and a subset of cases have been linked to dysfunction of Cav1.4 calcium channels and in turn compromised synaptic transmission. Here, we show that active zone proteins RIM1/2 are important regulators of Cav1.4 channel function in mouse rod photoreceptors and thus synaptic activity. The conditional double knock-out (cdko) of RIM1 and RIM2 from rods starting a few weeks after birth did not change Cav1.4 protein expression at rod ribbon synapses nor was the morphology of the ribbon altered. Heterologous overexpression of RIM2 with Cav1.4 had no significant influence on current density when examined with BaCl2 as the charge carrier. Nonetheless, whole-cell voltage-clamp recordings from cdko rods revealed a profound reduction in Ca(2+) currents. Concomitantly, we observed a 4-fold reduction in spontaneous miniature release events from the cdko rod terminals and an almost complete absence of evoked responses when monitoring changes in membrane incorporation after strong step depolarizations. Under control conditions, 49 and 83 vesicles were released with 0.2 and 1 s depolarizations, respectively, which is close to the maximal number of vesicles estimated to be docked at the base of the ribbon active zone, but without RIM1/2, only a few vesicles were stimulated for release after a 1 s stimulation. In conclusion, our study shows that RIM1/2 potently enhance the influx of Ca(2+) into rod terminals through Cav1.4 channels, which is vitally important for the release of vesicles from the rod ribbon. Significance statement: Active zone scaffolding proteins are thought to bring multiple components involved in Ca(2+)-dependent exocytosis into functional interactions. We show that removal of scaffolding proteins RIM1/2 from rod photoreceptor ribbon synapses causes a dramatic loss of Ca(2+) influx through Cav1.4 channels and a correlated reduction in evoked release, yet the channels remain localized to synaptic ribbons in a normal fashion. Our findings strongly argue that RIM1/2 facilitate Ca(2+) entry and in turn Ca(2+) evoked release by modulating Cav1.4 channel openings; however, RIM1/2 are not needed for the retention of Cav1.4 at the synapse. In summary, a key function of RIM1/2 at rod ribbons is to enhance Cav1.4 channel activity, possibly through direct or indirect modulation of the channel.

Ribbon Synapses and Visual Processing in the Retina

The first synapses transmitting visual information contain an unusual organelle, the ribbon, which is involved in the transport and priming of vesicles to be released at the active zone. The ribbon is one of many design features that allow efficient refilling of the active zone, which in turn enables graded changes in membrane potential to be transmitted using a continuous mode of neurotransmitter release. The ribbon also plays a key role in supplying vesicles for rapid and transient bursts of release that signal fast changes, such as the onset of light. We increasingly understand how the physiological properties of ribbon synapses determine basic transformations of the visual signal and, in particular, how the process of refilling the active zone regulates the gain and adaptive properties of the retinal circuit. The molecular basis of ribbon function is, however, far from clear.

Novel somatic mutations and distinct molecular signature in aldosterone-producing adenomas

Aldosterone-producing adenomas (APAs) are found in 1.5-3.0% of hypertensive patients in primary care and can be cured by surgery. Elucidation of genetic events may improve our understanding of these tumors and ultimately improve patient care. Approximately 40% of APAs harbor a missense mutation in the KCNJ5 gene. More recently, somatic mutations in CACNA1D, ATP1A1 and ATP2B3, also important for membrane potential/intracellular Ca(2) (+) regulation, were observed in APAs. In this study, we analyzed 165 APAs for mutations in selected regions of these genes. We then correlated mutational findings with clinical and molecular phenotype using transcriptome analysis, immunohistochemistry and semiquantitative PCR. Somatic mutations in CACNA1D in 3.0% (one novel mutation), ATP1A1 in 6.1% (six novel mutations) and ATP2B3 in 3.0% (two novel mutations) were detected. All observed mutations were located in previously described hotspot regions. Patients with tumors harboring mutations in CACNA1D, ATP1A1 and ATP2B3 were operated at an older age, were more often male and had tumors that were smaller than those in patients with KCNJ5 mutated tumors. Microarray transcriptome analysis segregated KCNJ5 mutated tumors from ATP1A1/ATP2B3 mutated tumors and those without mutation. We observed significant transcription upregulation of CYP11B2, as well as the previously described glomerulosa-specific gene NPNT, in ATP1A1/ATP2B3 mutated tumors compared to KCNJ5 mutated tumors. In summary, we describe novel somatic mutations in proteins regulating the membrane potential/intracellular Ca(2) (+) levels, and also a distinct mRNA and clinical signature, dependent on genetic alteration.

Sense and specificity in neuronal calcium signalling

Changes in the intracellular free calcium concentration ([Ca²⁺]i) in neurons regulate many and varied aspects of neuronal function over time scales from microseconds to days. The mystery is how a single signalling ion can lead to such diverse and specific changes in cell function. This is partly due to aspects of the Ca²⁺ signal itself, including its magnitude, duration, localisation and persistent or oscillatory nature. The transduction of the Ca²⁺ signal requires Ca²⁺binding to various Ca²⁺ sensor proteins. The different properties of these sensors are important for differential signal processing and determine the physiological specificity of Ca(2+) signalling pathways. A major factor underlying the specific roles of particular Ca²⁺ sensor proteins is the nature of their interaction with target proteins and how this mediates unique patterns of regulation. We review here recent progress from structural analyses and from functional analyses in model organisms that have begun to reveal the rules that underlie Ca²⁺ sensor protein specificity for target interaction. We discuss three case studies exemplifying different aspects of Ca²⁺ sensor/target interaction. This article is part of a special issue titled the 13th European Symposium on Calcium.

Spectrum of Cav1.4 dysfunction in congenital stationary night blindness type 2

Defective retinal synaptic transmission in patients affected with congenital stationary night blindness type 2 (CSNB2) can result from different dysfunction phenotypes in Cav1.4 L-type calcium channels. Here we investigated two prototypical Cav1.4 variants from either end of the functional spectrum. Using whole-cell and single-channel patch-clamp techniques, we provide analysis of the biophysical characteristics of the point mutation L860P and the C-terminal truncating mutation R1827X. L860P showed a typical loss-of-function phenotype attributed to a reduced number of functional channels expressed at the plasma membrane as implied by gating current and non-stationary noise analyses. This phenotype can be rationalized, because the inserted proline is predicted to break an amphipatic helix close to the transmembrane segment IIIS1 and thus to reduce channel stability and promote misfolding. In fact, L860P was subject to an increased turnover. In contrast, R1827X displayed an apparent gain-of-function phenotype, i.e., due to a hyperpolarizing shift of the IV-curve and increased single-channel activity. However, truncation also resulted in the loss of functional C-terminal modulation and thus unmasked calcium-dependent inactivation. Thus R1827X failed to support continuous calcium influx. Current inactivation curtails the dynamic range of photoreceptors (e.g., when adapting to variation in illumination). Taken together, the analysis of two representative mutations that occur in CSNB2 patients revealed fundamental differences in the underlying defect. These may explain subtle variations in the clinical manifestation and must be taken into account, if channel function is to be restored by pharmacochaperones or related approaches.

Cav1.4 IT mouse as model for vision impairment in human congenital stationary night blindness type 2

Mutations in the CACNA1F gene encoding the Cav1.4 Ca (2+) channel are associated with X-linked congenital stationary night blindness type 2 (CSNB2). Despite the increasing knowledge about the functional behavior of mutated channels in heterologous systems, the pathophysiological mechanisms that result in vision impairment remain to be elucidated. This work provides a thorough functional characterization of the novel IT mouse line that harbors the gain-of-function mutation I745T reported in a New Zealand CSNB2 family. (1) Electroretinographic recordings in IT mice permitted a direct comparison with human data. Our data supported the hypothesis that a hyperpolarizing shift in the voltage-dependence of channel activation-as seen in the IT gain-of-function mutant (2)-may reduce the dynamic range of photoreceptor activity. Morphologically, the retinal outer nuclear layer in adult IT mutants was reduced in size and cone outer segments appeared shorter. The organization of the outer plexiform layer was disrupted, and synaptic structures of photoreceptors had a variable, partly immature, appearance. The associated visual deficiency was substantiated in behavioral paradigms. The IT mouse line serves as a specific model for the functional phenotype of human CSNB2 patients with gain-of-function mutations and may help to further understand the dysfunction in CSNB.

Somatic mutations in ATP1A1 and CACNA1D underlie a common subtype of adrenal hypertension

At least 5% of individuals with hypertension have adrenal aldosterone-producing adenomas (APAs). Gain-of-function mutations in KCNJ5 and apparent loss-of-function mutations in ATP1A1 and ATP2A3 were reported to occur in APAs. We find that KCNJ5 mutations are common in APAs resembling cortisol-secreting cells of the adrenal zona fasciculata but are absent in a subset of APAs resembling the aldosterone-secreting cells of the adrenal zona glomerulosa. We performed exome sequencing of ten zona glomerulosa-like APAs and identified nine with somatic mutations in either ATP1A1, encoding the Na(+)/K(+) ATPase α1 subunit, or CACNA1D, encoding Cav1.3. The ATP1A1 mutations all caused inward leak currents under physiological conditions, and the CACNA1D mutations induced a shift of voltage-dependent gating to more negative voltages, suppressed inactivation or increased currents. Many APAs with these mutations were <1 cm in diameter and had been overlooked on conventional adrenal imaging. Recognition of the distinct genotype and phenotype for this subset of APAs could facilitate diagnosis.

Somatic and germline CACNA1D calcium channel mutations in aldosterone-producing adenomas and primary aldosteronism

Adrenal aldosterone-producing adenomas (APAs) constitutively produce the salt-retaining hormone aldosterone and are a common cause of severe hypertension. Recurrent mutations in the potassium channel KCNJ5 that result in cell depolarization and Ca²⁺ influx cause ~40% of these tumors¹. We found five somatic mutations (four altering glycine 403, one altering isoleucine 770) in CACNA1D, encoding a voltage-gated calcium channel, among 43 non-KCNJ5-mutant APAs. These mutations lie in S6 segments that line the channel pore. Both result in channel activation at less depolarized potentials, and glycine 403 mutations also impair channel inactivation. These effects are inferred to cause increased Ca²⁺ influx, the sufficient stimulus for aldosterone production and cell proliferation in adrenal glomerulosa². Remarkably, we identified de novo mutations at the identical positions in two children with a previously undescribed syndrome featuring primary aldosteronism and neuromuscular abnormalities. These findings implicate gain of function Ca²⁺ channel mutations in aldosterone-producing adenomas and primary aldosteronism.

What can naturally occurring mutations tell us about Ca(v)1.x channel function?

Voltage-gated Ca^2 + channels allow for Ca^2 +-dependent intracellular signaling by directly mediating Ca^2 + ion influx, by physical coupling to intracellular Ca^2 + release channels or functional coupling to other ion channels such as Ca^2 + activated potassium channels. L-type Ca^2 + channels that comprise the family of Ca_v1 channels are expressed in many electrically excitable tissues and are characterized by their unique sensitivity to dihydropyridines. In this issue, we summarize genetic defects in L-type Ca^2 + channels and analyze their role in human diseases (Ca^2 + channelopathies); e.g. mutations in Ca_v1.2 α1 cause Timothy and Brugada syndrome, mutations in Ca_v1.3 α1 are linked to sinoatrial node dysfunction and deafness while mutations in Ca_v1.4 α1 are associated with X-linked retinal disorders such as an incomplete form of congenital stationary night blindness. Herein, we also put the mutations underlying the channel's dysfunction into the structural context of the pore-forming α1 subunit. This analysis highlights the importance of combining functional data with structural analysis to gain a deeper understanding for the disease pathophysiology as well as for physiological channel function. This article is part of a Special Issue entitled: Calcium channels.

Levels of Ca(V)1.2 L-Type Ca(2+) Channels Peak in the First Two Weeks in Rat Hippocampus Whereas Ca(V)1.3 Channels Steadily Increase through Development

Influx of calcium through voltage-dependent channels regulates processes throughout the nervous system. Specifically, influx through L-type channels plays a variety of roles in early neuronal development and is commonly modulated by G-protein-coupled receptors such as GABA(B) receptors. Of the four isoforms of L-type channels, only Ca(V)1.2 and Ca(V)1.3 are predominately expressed in the nervous system. Both isoforms are inhibited by the same pharmacological agents, so it has been difficult to determine the role of specific isoforms in physiological processes. In the present study, Western blot analysis and confocal microscopy were utilized to study developmental expression levels and patterns of Ca(V)1.2 and Ca(V)1.3 in the CA1 region of rat hippocampus. Steady-state expression of Ca(V)1.2 predominated during the early neonatal period decreasing by day 12. Steady-state expression of Ca(V)1.3 was low at birth and gradually rose to adult levels by postnatal day 15. In immunohistochemical studies, antibodies against Ca(V)1.2 and Ca(V)1.3 demonstrated the highest intensity of labeling in the proximal dendrites at all ages studied (P1-72). Immunohistochemical studies on one-week-old hippocampi demonstrated significantly more colocalization of GABA(B) receptors with Ca(V)1.2 than with Ca(V)1.3, suggesting that modulation of L-type calcium current in early development is mediated through Ca(V)1.2 channels.

Mutation screening of TRPM1, GRM6, NYX and CACNA1F genes in patients with congenital stationary night blindness

The aim of this study was to identify mutations in the TRPM1, GRM6, NYX and CACNA1F genes in patients with congenital stationary night blindness (CSNB). Twenty-four unrelated patients with CSNB were ascertained. Sanger sequencing was used to analyze the coding exons and adjacent intronic regions of TRPM1, GRM6, NYX and CACNA1F. Six mutations were identified in six unrelated patients, including five novel and one known. Of the six, three novel hemizygous mutations, c.92G>A (p.Cys31Tyr), c.149G>C (p.Ary50Pro), and c.[272T>A;1429G>C] (p.[Leu91Gln;Gly477Arg]), were found in NYX in three patients, respectively. A novel c.[1984_1986delCTC;3001G>A] (p.[Leu662del;Gly1001Arg]) mutation was detected in CACNA1F in one patient. One novel and one known heterozygous variation, c.1267T>C (p.Cys423Arg) and c.1537G>A (p.Val513Met), were detected in GRM6 in two patients, respectively. No variations were found in TRPM1. The results expand the mutation spectrum of NYX, CACNA1F and GRM6. They also suggest that NYX mutations are a common cause of CSNB.

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.

Congenital stationary night blindness in mice - a tale of two Cacna1f mutants

BACKGROUND

Mutations in CACNA1F, which encodes the Ca(v)1.4 subunit of a voltage-gated L-type calcium channel, cause X-linked incomplete congenital stationary night blindness (CSNB2), a condition of defective retinal neurotransmission which results in night blindness, reduced visual acuity, and diminished ERG b-wave. We have characterized two putative murine CSNB2 models: an engineered null-mutant, with a stop codon (G305X); and a spontaneous mutant with an ETn insertion in intron 2 of Cacna1f (nob2).

METHODS

Cacna1f ( G305X ): Adults were characterized by visual function (photopic optokinetic response, OKR); gene expression (microarray) and by cell death (TUNEL) and synaptic development (TEM). Cacna1f ( nob2 ): Adults were characterized by properties of Cacna1f mRNA (cloning and sequencing) and expressed protein (immunoblotting, electrophysiology, filamin [cytoskeletal protein] binding), and OKR.

RESULTS

The null mutation in Cacna1f ( G305X ) mice caused loss of cone cell ribbons, failure of OPL synaptogenesis, ERG b-wave and absence of OKR. In Cacna1f ( nob2 ) mice alternative ETn splicing produced ~90% Cacna1f mRNA having a stop codon, but ~10% mRNA encoding a complete polypeptide. Cacna1f ( nob2 ) mice had normal OKR, and alternatively-spliced complete protein had WT channel properties, but alternative ETn splicing abolished N-terminal protein binding to filamin.

CONCLUSIONS

Ca(v)1.4 plays a key role in photoreceptor synaptogenesis and synaptic function in mouse retina. Cacna1f ( G305X ) is a true knockout model for human CSNB2, with prominent defects in cone and rod function. Cacna1f ( nob2 ) is an incomplete knockout model for CSNB2, because alternative splicing in an ETn element leads to some full-length Ca(v)1.4 protein, and some cones surviving to drive photopic visual responses.

Calcium binding protein-mediated regulation of voltage-gated calcium channels linked to human diseases

Calcium ion entry through voltage-gated calcium channels is essential for cellular signalling in a wide variety of cells and multiple physiological processes. Perturbations of voltage-gated calcium channel function can lead to pathophysiological consequences. Calcium binding proteins serve as calcium sensors and regulate the calcium channel properties via feedback mechanisms. This review highlights the current evidences of calcium binding protein-mediated channel regulation in human diseases.

Hallmarks of the channelopathies associated with L-type calcium channels: a focus on the Timothy mutations in Ca(v)1.2 channels

Within the voltage-gated calcium channels (Cav channels) family, there are four genes coding for the L-type Cav channels (Cav1). The Cav1 channels underly many important physiological functions like excitation-contraction coupling, hormone secretion, neuronal excitability and gene transcription. Mutations found in the genes encoding the Cav channels define a wide variety of diseases called calcium channelopathies and all four genes coding the Cav1 channels are carrying such mutations. L-type calcium channelopathies include muscular, neurological, cardiac and vision syndromes. Among them, the Timothy syndrome (TS) is linked to missense mutations in CACNA1C, the gene that encodes the Ca(v)1.2 subunit. Here we review the important features of the Cav1 channelopathies. We also report on the specific properties of TS-Ca(v)1.2 channels, which display non-inactivating calcium current as well as higher plasma membrane expression. Overall, we conclude that both electrophysiological and surface expression properties must be investigated to better account for the functional consequences of mutations linked to calcium channelopathies.

Voltage-gated calcium channels and disease

Voltage-gated calcium channels are a family of integral membrane calcium-selective proteins found in all excitable and many nonexcitable cells. Calcium influx affects membrane electrical properties by depolarizing cells and generally increasing excitability. Calcium entry further regulates multiple intracellular signaling pathways as well as the biochemical factors that mediate physiological functions such as neurotransmitter release and muscle contraction. Small changes in the biophysical properties or expression of calcium channels can result in pathophysiological changes leading to serious chronic disorders. In humans, mutations in calcium channel genes have been linked to a number of serious neurological, retinal, cardiac, and muscular disorders.

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.

Molecular endpoints of Ca2+/calmodulin- and voltage-dependent inactivation of Ca(v)1.3 channels

Ca²⁺/calmodulin- and voltage-dependent inactivation (CDI and VDI) comprise vital prototypes of Ca²⁺ channel modulation, rich with biological consequences. Although the events initiating CDI and VDI are known, their downstream mechanisms have eluded consensus. Competing proposals include hinged-lid occlusion of channels, selectivity filter collapse, and allosteric inhibition of the activation gate. Here, novel theory predicts that perturbations of channel activation should alter inactivation in distinctive ways, depending on which hypothesis holds true. Thus, we systematically mutate the activation gate, formed by all S6 segments within Ca_V1.3. These channels feature robust baseline CDI, and the resulting mutant library exhibits significant diversity of activation, CDI, and VDI. For CDI, a clear and previously unreported pattern emerges: activation-enhancing mutations proportionately weaken inactivation. This outcome substantiates an allosteric CDI mechanism. For VDI, the data implicate a “hinged lid–shield” mechanism, similar to a hinged-lid process, with a previously unrecognized feature. Namely, we detect a “shield” in Ca_V1.3 channels that is specialized to repel lid closure. These findings reveal long-sought downstream mechanisms of inactivation and may furnish a framework for the understanding of Ca²⁺ channelopathies involving S6 mutations.

Expression and 1,4-dihydropyridine-binding properties of brain L-type calcium channel isoforms

The L-type calcium channel (LTCC) isoforms Ca(v)1.2 and Ca(v)1.3 display similar 1,4-dihydropyridine (DHP) binding properties and are both expressed in mammalian brain. Recent work implicates Ca(v)1.3 channels as interesting drug targets, but no isoform-selective modulators exist. It is also unknown to what extent Ca(v)1.1 and Ca(v)1.4 contribute to L-type-specific DHP binding activity in brain. To address this question and to determine whether DHPs can discriminate between Ca(v)1.2 and Ca(v)1.3 binding pockets, we combined radioreceptor assays and quantitative polymerase chain reaction (qPCR). We bred double mutants (Ca(v)-DM) from mice expressing mutant Ca(v)1.2 channels [Ca(v)1.2DHP(-/-)] lacking high affinity for DHPs and from Ca(v)1.3 knockouts [Ca(v)1.3(-/-)]. (+)-[(3)H]isradipine binding to Ca(v)1.2DHP(-/-) and Ca(v)-DM brains was reduced to 15.1 and 4.4% of wild type, respectively, indicating that Ca(v)1.3 accounts for 10.7% of brain LTCCs. qPCR revealed that Ca(v)1.1 and Ca(v)1.4 alpha(1) subunits comprised 0.08% of the LTCC transcripts in mouse whole brain, suggesting that they cannot account for the residual binding. Instead, this could be explained by low-affinity binding (127-fold K(d) increase) to the mutated Ca(v)1.2 channels. Inhibition of (+)-[(3)H]isradipine binding to Ca(v)1.2DHP(-/-) (predominantly Ca(v)1.3) and wild-type (predominantly Ca(v)1.2) brain membranes by unlabeled DHPs revealed a 3- to 4-fold selectivity of nitrendipine and nifedipine for the Ca(v)1.2 binding pocket, a finding further confirmed with heterologously expressed channels. This suggests that small differences in their binding pockets may allow development of isoform-selective modulators for LTCCs and that, because of their very low expression, Ca(v)1.1 and Ca(v)1.4 are unlikely to serve as drug targets to treat CNS diseases.

Modified Ca(v)1.4 expression in the Cacna1f(nob2) mouse due to alternative splicing of an ETn inserted in exon 2

The Cacna1f^nob2 mouse is reported to be a naturally occurring null mutation for the Ca_v1.4 calcium channel gene and the phenotype of this mouse is not identical to that of the targeted gene knockout model. We found two mRNA species in the Cacna1f^nob2 mouse: approximately 90% of the mRNA represents a transcript with an in-frame stop codon within exon 2 of CACNA1F, while approximately 10% of the mRNA represents a transcript in which alternative splicing within the ETn element has removed the stop codon. This latter mRNA codes for full length Ca_v1.4 protein, detectable by Western blot analysis that is predicted to differ from wild type Ca_v1.4 protein in a region of approximately 22 amino acids in the N-terminal portion of the protein. Electrophysiological analysis with either mouse Ca_v1.4^wt or Ca_v1.4^nob2 cDNA revealed that the alternatively spliced protein does not differ from wild type with respect to activation and inactivation characteristics; however, while the wild type N-terminus interacted with filamin proteins in a biochemical pull-down experiment, the alternatively spliced N-terminus did not. The Cacna1f^nob2 mouse electroretinogram displayed reduced b-wave and oscillatory potential amplitudes, and the retina was morphologically disorganized, with substantial reduction in thickness of the outer plexiform layer and sprouting of bipolar cell dendrites ectopically into the outer nuclear layer. Nevertheless, the spatial contrast sensitivity (optokinetic response) of Cacna1f^nob2 mice was generally similar to that of wild type mice. These results suggest the Cacna1f^nob2 mouse is not a CACNA1F knockout model. Rather, alternative splicing within the ETn element can lead to full-length Ca_v1.4 protein, albeit at reduced levels, and the functional Ca_v1.4 mutant may be incapable of interacting with cytoskeletal filamin proteins. These changes, do not alter the ability of the Cacna1f^nob2 mouse to detect and follow moving sine-wave gratings compared to their wild type counterparts.