Structural insights into activation of the retinal L-type Ca²⁺ channel (Cav1.4) by Ca²⁺-binding protein 4 (CaBP4)

Unusual OCT findings in a patient with CABP4-associated cone-rod synaptic disorder

PURPOSE

Bi-allelic variants in CABP4 are associated with congenital cone-rod synaptic disorder, which has also been classified, electrophysiologically, as incomplete congenital stationary night blindness (iCSNB). We describe clinical findings in a patient who demonstrated an unusual macular optical coherence tomography (OCT) phenotype, not previously reported in this condition.

METHODS

Our patient underwent multimodal retinal imaging, international standard full-field ERG testing and whole genome sequencing.

RESULTS

The patient was a 60-year-old woman with non-progressive visual impairment since birth, nystagmus and preference for dim lighting. Clinical fundus examination was unremarkable. OCT imaging revealed a hypo-reflective zone under an elevated fovea in both eyes. ERGs showed an electronegative DA10 response, with severely abnormal light-adapted responses. Whole genome sequencing revealed homozygosity for a known pathogenic variant in CABP4. No variants were found in other genes that could explain the patient's phenotype.

CONCLUSIONS

OCT findings of foveal elevation and an underlying hypo-reflective zone are novel in this condition. Whilst the clinical history was similar to achromatopsia and other cone dysfunction syndromes, ERG findings suggested disease associated with CACNA1F or CABP4. As CACNA1F is X-linked, CABP4 was more likely, and confirmed on genetic testing. The patient saw better in dim light, confirming that night blindness is not a feature of CABP4-associated disease. Our case highlights the value of ERGs in discriminating between causes of cone dysfunction, and extends the range of retinal imaging phenotypes reported in this disorder.

Calmodulin promotes a Ca2+ -dependent conformational change in the C-terminal regulatory domain of CaV 1.2

Calmodulin (CaM) binds to the membrane-proximal cytosolic C-terminal domain of CaV 1.2 (residues 1520-1669, CT(1520-1669)) and causes Ca2+ -induced conformational changes that promote Ca2+ -dependent channel inactivation (CDI). We report biophysical studies that probe the structural interaction between CT(1520-1669) and CaM. The recombinantly expressed CT(1520-1669) is insoluble, but can be solubilized in the presence of Ca2+ -saturated CaM (Ca4 /CaM), but not in the presence of Ca2+ -free CaM (apoCaM). We show that half-calcified CaM (Ca2 /CaM12 ) forms a complex with CT(1520-1669) that is less soluble than CT(1520-1669) bound to Ca4 /CaM. The NMR spectrum of CT(1520-1669) reveals spectral differences caused by the binding of Ca2 /CaM12 versus Ca4 /CaM, suggesting that the binding of Ca2+ to the CaM N-lobe may induce a conformational change in CT(1520-1669).

A novel missense creatine mutant of CaBP4, c.464G>A (p.G155D), associated with autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE), reduces the expression of CaBP4

BACKGROUND

CaBP4 encodes Ca²⁺-binding protein 4, a neuronal Ca²⁺-binding protein that participates in many cellular processes by regulating the concentration of free Ca²⁺ ions. De novo CaBP4 variants have been identified as a cause of congenital stationary night blindness (CSNB). However, we recently reported a 4-generation pedigree with 11 individuals diagnosed with autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) that were validated with only one novel missense mutation, c.464G>A (p.G155D), in CaBP4. De novo CaBP4 variants have never been reported to be related with ADNFLE. This study aimed to identify whether c.464G>A (p.G155D) in CaBP4 reduced the expression of CaBP4.

METHODS

In vitro experiments using recombinant protein expressed in human neuron cells were utilized in this study. Real-time polymerase chain reaction (RT-PCR) was performed to evaluate the effect of c.464G>A on CaBP4 mRNA expression. Western blot was performed to assess the effect of c.464G>A on CaBP4 protein expression.

RESULTS

According to the RT-PCR and Western blot results, c.464G>A (p.G155D) was associated with an increased expression of CaBP4 mRNA and a reduced expression of CaBP4 protein.

CONCLUSIONS

These results reveal that c.464G>A (p.G155D) in CaBP4 reduced the expression of CaBP4 by reducing the stability of the CaBP4 protein. Mutations in the CaBP4 gene may be associated with ADNFLE.

NMR and EPR-DEER Structure of a Dimeric Guanylate Cyclase Activator Protein-5 from Zebrafish Photoreceptors

Retinal guanylate cyclases (RetGCs) are regulated by a family of guanylate cyclase-activating proteins (called GCAP1-7). GCAPs form dimers that bind to Ca²⁺ and confer Ca²⁺ sensitive activation of RetGC during visual phototransduction. The GCAP5 homologue from zebrafish contains two nonconserved cysteine residues (Cys15 and Cys17) that bind to ferrous ion, which stabilizes GCAP5 dimerization and diminishes its ability to activate RetGC. Here, we present NMR and EPR-DEER structural analysis of a GCAP5 dimer in the Mg²⁺-bound, Ca²⁺-free, Fe²⁺-free activator state. The NMR-derived structure of GCAP5 is similar to the crystal structure of Ca²⁺-bound GCAP1 (root-mean-square deviation of 2.4 Å), except that the N-terminal helix of GCAP5 is extended by two residues, which allows the sulfhydryl groups of Cys15 and Cys17 to become more solvent exposed in GCAP5 to facilitate Fe²⁺ binding. Nitroxide spin-label probes were covalently attached to particular cysteine residues engineered in GCAP5: C15, C17, T26C, C28, N56C, C69, C105, N139C, E152C, and S159C. The intermolecular distance of each spin-label probe in dimeric GCAP5 (measured by EPR-DEER) defined restraints for calculating the dimer structure by molecular docking. The GCAP5 dimer possesses intermolecular hydrophobic contacts involving the side chain atoms of H18, Y21, M25, F72, V76, and W93, as well as an intermolecular salt bridge between R22 and D71. The structural model of the GCAP5 dimer was validated by mutations (H18E/Y21E, H18A/Y21A, R22D, R22A, M25E, D71R, F72E, and V76E) at the dimer interface that disrupt dimerization of GCAP5 and affect the activation of RetGC. We propose that GCAP5 dimerization may play a role in the Fe²⁺-dependent regulation of cyclase activity in zebrafish photoreceptors.

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.

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.

Voltage- and calcium-gated ion channels of neurons in the vertebrate retina

In this review, we summarize studies investigating the types and distribution of voltage- and calcium-gated ion channels in the different classes of retinal neurons: rods, cones, horizontal cells, bipolar cells, amacrine cells, interplexiform cells, and ganglion cells. We discuss differences among cell subtypes within these major cell classes, as well as differences among species, and consider how different ion channels shape the responses of different neurons. For example, even though second-order bipolar and horizontal cells do not typically generate fast sodium-dependent action potentials, many of these cells nevertheless possess fast sodium currents that can enhance their kinetic response capabilities. Ca2+ channel activity can also shape response kinetics as well as regulating synaptic release. The L-type Ca2+ channel subtype, CaV1.4, expressed in photoreceptor cells exhibits specific properties matching the particular needs of these cells such as limited inactivation which allows sustained channel activity and maintained synaptic release in darkness. The particular properties of K+ and Cl- channels in different retinal neurons shape resting membrane potentials, response kinetics and spiking behavior. A remaining challenge is to characterize the specific distributions of ion channels in the more than 100 individual cell types that have been identified in the retina and to describe how these particular ion channels sculpt neuronal responses to assist in the processing of visual information by the retina.

Calcium Sensors in Neuronal Function and Dysfunction

Calcium signaling in neurons as in other cell types can lead to varied changes in cellular function. Neuronal Ca²⁺ signaling processes have also become adapted to modulate the function of specific pathways over a wide variety of time domains and these can have effects on, for example, axon outgrowth, neuronal survival, and changes in synaptic strength. Ca²⁺ also plays a key role in synapses as the trigger for fast neurotransmitter release. Given its physiological importance, abnormalities in neuronal Ca²⁺ signaling potentially underlie many different neurological and neurodegenerative diseases. The mechanisms by which changes in intracellular Ca²⁺ concentration in neurons can bring about diverse responses is underpinned by the roles of ubiquitous or specialized neuronal Ca²⁺ sensors. It has been established that synaptotagmins have key functions in neurotransmitter release, and, in addition to calmodulin, other families of EF-hand-containing neuronal Ca²⁺ sensors, including the neuronal calcium sensor (NCS) and the calcium-binding protein (CaBP) families, play important physiological roles in neuronal Ca²⁺ signaling. It has become increasingly apparent that these various Ca²⁺ sensors may also be crucial for aspects of neuronal dysfunction and disease either indirectly or directly as a direct consequence of genetic variation or mutations. An understanding of the molecular basis for the regulation of the targets of the Ca²⁺ sensors and the physiological roles of each protein in identified neurons may contribute to future approaches to the development of treatments for a variety of human neuronal disorders.

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.

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.

Exome sequencing identified a novel missense mutation c.464G>A (p.G155D) in Ca2+-binding protein 4 (CABP4) in a Chinese pedigree with autosomal dominant nocturnal frontal lobe epilepsy

The aim of this study was to identify disease-causing gene mutations in a Chinese family affected with autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE), a 4-generation pedigree of 27 members in the Southern Chinese Han population, including 11 individuals diagnosed with ADNFLE. DNA samples were collected from 15 family members, chinese han people, including seven affected and eight unaffected individuals. None of these patients had night blindness or visual disorders. Four affected individuals were screened for mutations using whole-exome sequencing, and 13 potentially interesting mutations shared by all the four affected individuals were validated using the Sanger sequencing method. Only one novel missense mutation c.464G>A (p.G155D) in the CABP4 gene, encoding the neuronal Ca²⁺-binding protein 4 (CaBP4), was present in all seven affected individuals in this family as revealed by PCR with blood DNA samples using CABP4 primers. The mutation was also found in one young unaffected family member, but was absent from 300 unrelated control subjects. The p.G155D mutation, located near the Ca²⁺ binding motif EF-hand 1 and the L-type Ca²⁺ channel (Cav1.4) binding motif within the N-terminal lobe of CaBP4, is predicted to affect protein function according to the bioinformatics tools PolyPhen-2 and SIFT. These findings suggest that mutations in the CABP4 gene may be linked to ADNFLE.

Structure of Guanylyl Cyclase Activator Protein 1 (GCAP1) Mutant V77E in a Ca2+-free/Mg2+-bound Activator State

GCAP1, a member of the neuronal calcium sensor subclass of the calmodulin superfamily, confers Ca(2+)-sensitive activation of retinal guanylyl cyclase 1 (RetGC1). We present NMR resonance assignments, residual dipolar coupling data, functional analysis, and a structural model of GCAP1 mutant (GCAP1(V77E)) in the Ca(2+)-free/Mg(2+)-bound state. NMR chemical shifts and residual dipolar coupling data reveal Ca(2+)-dependent differences for residues 170-174. An NMR-derived model of GCAP1(V77E) contains Mg(2+) bound at EF2 and looks similar to Ca(2+) saturated GCAP1 (root mean square deviations = 2.0 Å). Ca(2+)-dependent structural differences occur in the fourth EF-hand (EF4) and adjacent helical region (residues 164-174 called the Ca(2+) switch helix). Ca(2+)-induced shortening of the Ca(2+) switch helix changes solvent accessibility of Thr-171 and Leu-174 that affects the domain interface. Although the Ca(2+) switch helix is not part of the RetGC1 binding site, insertion of an extra Gly residue between Ser-173 and Leu-174 as well as deletion of Arg-172, Ser-173, or Leu-174 all caused a decrease in Ca(2+) binding affinity and abolished RetGC1 activation. We conclude that Ca(2+)-dependent conformational changes in the Ca(2+) switch helix are important for activating RetGC1 and provide further support for a Ca(2+)-myristoyl tug mechanism.