α2δ-4 Is Required for the Molecular and Structural Organization of Rod and Cone Photoreceptor Synapses

α2δ-4 is an auxiliary subunit of voltage-gated Cav1.4 L-type channels that regulate the development and mature exocytotic function of the photoreceptor ribbon synapse. In humans, mutations in the CACNA2D4 gene encoding α2δ-4 cause heterogeneous forms of vision impairment in humans, the underlying pathogenic mechanisms of which remain unclear. To investigate the retinal function of α2δ-4, we used genome editing to generate an α2δ-4 knock-out (α2δ-4 KO) mouse. In male and female α2δ-4 KO mice, rod spherules lack ribbons and other synaptic hallmarks early in development. Although the molecular organization of cone synapses is less affected than rod synapses, horizontal and cone bipolar processes extend abnormally in the outer nuclear layer in α2δ-4 KO retina. In reconstructions of α2δ-4 KO cone pedicles by serial block face scanning electron microscopy, ribbons appear normal, except that less than one-third show the expected triadic organization of processes at ribbon sites. The severity of the synaptic defects in α2δ-4 KO mice correlates with a progressive loss of Cav1.4 channels, first in terminals of rods and later cones. Despite the absence of b-waves in electroretinograms, visually guided behavior is evident in α2δ-4 KO mice and better under photopic than scotopic conditions. We conclude that α2δ-4 plays an essential role in maintaining the structural and functional integrity of rod and cone synapses, the disruption of which may contribute to visual impairment in humans with CACNA2D4 mutations.SIGNIFICANCE STATEMENT In the retina, visual information is first communicated by the synapse formed between photoreceptors and second-order neurons. The mechanisms that regulate the structural integrity of this synapse are poorly understood. Here we demonstrate a role for α2δ-4, a subunit of voltage-gated Ca2+ channels, in organizing the structure and function of photoreceptor synapses. We find that presynaptic Ca2+ channels are progressively lost and that rod and cone synapses are disrupted in mice that lack α2δ-4. Our results suggest that alterations in presynaptic Ca2+ signaling and photoreceptor synapse structure may contribute to vision impairment in humans with mutations in the CACNA2D4 gene encoding α2δ-4.

CaBP1 regulates Cav1 L-type Ca2+ channels and their coupling to neurite growth and gene transcription in mouse spiral ganglion neurons

CaBP1 is a Ca²⁺ binding protein that is widely expressed in neurons in the brain, retina, and cochlea. In heterologous expression systems, CaBP1 interacts with and regulates voltage-gated Ca_v Ca²⁺ channels but whether this is the case in neurons is unknown. Here, we investigated the cellular functions of CaBP1 in cochlear spiral ganglion neurons (SGNs), which express high levels of CaBP1. Consistent with the role of CaBP1 as a suppressor of Ca²⁺-dependent inactivation (CDI) of Ca_v1 (L-type) channels, Ca_v1 currents underwent greater CDI in SGNs from mice lacking CaBP1 (C-KO) than in wild-type (WT) SGNs. The coupling of Ca_v1 channels to downstream signaling pathways was also disrupted in C-KO SGNs. Activity-dependent repression of neurite growth was significantly blunted and unresponsive to Ca_v1 antagonists in C-KO SGNs in contrast to WT SGNs. Moreover, Ca_v1-mediated Ca²⁺ signals and phosphorylation of cAMP-response element binding protein were reduced in C-KO SGNs compared to WT SGNs. Our findings establish a role for CaBP1 as an essential regulator of Ca_v1 channels in SGNs and their coupling to downstream pathways controlling activity-dependent transcription and neurite growth.

Phosphoproteomics study based on in vivo inhibition reveals sites of calmodulin-dependent protein kinase II regulation in the heart

BACKGROUND

The multifunctional Ca(2+)- and calmodulin-dependent protein kinase II (CaMKII) is a crucial mediator of cardiac physiology and pathology. Increased expression and activation of CaMKII has been linked to elevated risk for arrhythmic events and is a hallmark of human heart failure. A useful approach to determining CaMKII's role therein is large-scale analysis of phosphorylation events by mass spectrometry. However, current large-scale phosphoproteomics approaches have proved inadequate for high-fidelity identification of kinase-specific roles. The purpose of this study was to develop a phosphoproteomics approach to specifically identify CaMKII's downstream effects in cardiac tissue.

METHODS AND RESULTS

To identify putative downstream CaMKII targets in cardiac tissue, animals with myocardial-delimited expression of the specific peptide inhibitor of CaMKII (AC3-I) or an inactive control (AC3-C) were compared using quantitative phosphoproteomics. The hearts were isolated after isoproterenol injection to induce CaMKII activation downstream of β-adrenergic receptor agonist stimulation. Enriched phosphopeptides from AC3-I and AC3-C mice were differentially quantified using stable isotope dimethyl labeling, strong cation exchange chromatography and high-resolution LC-MS/MS. Phosphorylation levels of several hundred sites could be profiled, including 39 phosphoproteins noticeably affected by AC3-I-mediated CaMKII inhibition.

CONCLUSIONS

Our data set included known CaMKII substrates, as well as several new candidate proteins involved in functions not previously implicated in CaMKII signaling.

Nervous system function for the testis RNA-binding protein boule in Drosophila

The RNA-binding protein, Boule is conserved across species and is required for male fertility. Boule and the DAZ homologues in mice and humans appear specific to the testis. Boule functions in spermatogenesis by controlling the translation of the meiotic cell division cycle 25 (Cdc25) phosphatase, Twine. Here we show, for the first time, a function for the DAZ protein, Boule, outside of meiosis. We found that an isoform of Boule is expressed in the nervous system and when its expression is increased we observe mutant phenotypes in neural communication between the receptor and laminar cells of the fly eye, altered larval locomotion and when further overexpressed, viability. As in the germ line, genetic studies indicate that Boule functions in the Cdc25 phosphatase pathway in the nervous system. In a sensitized genetic background of Boule overexpression, we added a loss-of-function mutation of twine and demonstrated a role for Twine Cdc25, in the adult nervous system. Our results indicate that isoforms of boule are expressed outside of the male germ line and that these isoforms have a role in neural function, unlike the boule testis-specific isoform.

Genetic analysis of oxygen defense mechanisms in Drosophila melanogaster and identification of a novel behavioural mutant with a Shaker phenotype

Mutants of Drosophila melanogaster that lack Cu/Zn superoxide dismutase or urate are hypersensitive to reactive oxygen species (ROS) generated in vivo by the redox-cycling agent paraquat. We have subsequently employed paraquat as a selective agent to identify adult viable mutants potentially defective in other, perhaps unknown, components of ROS metabolism. Paraquat screening of ethyl methanesulfonate-induced second- and third-chromosome mutations yielded 24 paraquat hypersensitive mutants. Two mutants were identified as being new alleles of the previously identified doublesex (dsx) and pink (p) genes. The remainder of the mutations identified previously undescribed genes, including one second chromosome paraquat hypersensitive mutant that was found to exhibit shaking legs, abdomen pulsations, and body shuddering under ether anaesthesia. This recessive mutation was mapped to the polytene chromosome region of 48A5-48B2 and defines a new gene we named quiver (qvr). This mutation is similar in phenotype to the Shaker (Sh), ether-a-gogo (eag), and Hyperkinetic (Hk) mutations, all of which affect potassium channel function in D. melanogaster.