Mutation of the calcium channel gene Cacna1f disrupts calcium signaling, synaptic transmission and cellular organization in mouse retina

Abstract

Retinal neural transmission represents a key function of the eye. Identifying the molecular components of this vital process is helped by studies of selected human genetic eye disorders. For example, mutations in the calcium channel subunit gene CACNA1F cause incomplete X-linked congenital stationary night blindness (CSNB2 or iCSNB), a human retinal disorder with abnormal electrophysiological response and visual impairments consistent with a retinal neurotransmission defect. To understand the subcellular basis of this retinal disorder, we generated a mouse with a loss-of-function mutation by inserting a self-excising Cre-lox-neo cassette into exon 7 of the murine orthologue, Cacna1f. Electroretinography of the mutant mouse revealed a scotopic a-wave of marginally reduced amplitude compared with the wild-type mouse and absence of the post-receptoral b-wave and oscillatory potentials. Cone ERG responses together with visual evoked potentials and multi-unit activity in the superior colliculus were also absent. Calcium imaging in Fluo-4 loaded retinal slices depolarized with KCl showed 90% less peak signal in the photoreceptor synapses of the Cacna1f mutant than in wild-type mice. The absence of post-receptoral ERG responses and the diminished photoreceptor calcium signals are consistent with a loss of Ca((2+)) channel function in photoreceptors. Immunocytochemistry showed no detectable Ca(v)1.4 protein in the outer plexiform layer of Cacna1f-mutant mice, profound loss of photoreceptor synapses, and abnormal dendritic sprouting of second-order neurons in the photoreceptor layer. Together, these findings in the Cacna1f-mutant mouse reveal that the Ca(v)1.4 calcium channel is vital for the functional assembly and/or maintenance and synaptic functions of photoreceptor ribbon synapses. Moreover, the outcome of this study provides critical clues to the pathophysiology of the human retinal channelopathy of X-linked incomplete CSNB.

Thenob2mouse, a null mutation inCacna1f: Anatomical and functional abnormalities in the outer retina and their consequences on ganglion cell visual responses

“…It is not clear if these functional abnormalities noted in the α 1F -KO mutant are stable across age. Nevertheless, these functional abnormalities appear to distinguish α 1F -KO mice from patients with CSNB2 and raise the possibility that the α 1F -KO mouse develops cone photoreceptor dysfunction or degeneration, a possibility discussed by Mansergh et al (2005) and consistent with recent reports linking some Cacna1f mutations with progressive retinal disorders (Nakamura et al, 2003;Jalkanen et al, 2004;Hope et al, 2005).…”

Characterization of Cav1.4 Complexes (α11.4, β2, and α2δ4) in HEK293T Cells and in the Retina

“…Second, we identify the ␤ 2X13 splice variant, which is distinct from the ␤ 2a subunit expressed in the brain, as the major ␤ 2 subunit in human retina. Finally, we show that both (15) and in mice expressing a mutation in the ␣ 2 ␦ 4 gene, Cacna2d4, in which a truncated ␣ 2 ␦ 4 protein is expressed at very low levels compared with the full-length protein in wild-type mice (26). In the ␣ 2 ␦ 4 mutant mice, there is also delayed degeneration of cones, and human mutations in CACNA2D4 are associated with slowly progressive cone dystrophy (54).…”

“…Antibodies against Ca v 1.4 label both rod and cone terminals (11)(12)(13)(14). In mice, inactivation of CACNA1F, the gene encoding Ca v 1.4 (Ca v 1.4 KO), disrupts photoreceptor synaptic transmission and presynaptic calcium signaling (15) and prevents the maturation of photoreceptor synapses (13,16). In addition, human mutations in CACNA1F cause vision disorders, including incomplete congenital stationary night blindness 2, which is characterized by impaired rod photoreceptor transmission and low visual acuity in darkness (17)(18)(19)(20).…”

Complex Regulation of Voltage-dependent Activation and Inactivation Properties of Retinal Voltage-gated Cav1.4 L-type Ca2+ Channels by Ca2+-binding Protein 4 (CaBP4)

“…This retinal disease is characterized by loss of neurotransmission from rods to second order bipolar cells, which is attributable to a loss of Cav1.4. These findings are corroborated by the analysis of a genetic mouse model deficient for the Cav1.4 ␣ 1 subunit (8,9). In addition, in the mouse, deletion of another component of the Cav1.4 channel complex, the auxiliary ␤ 2a subunit, also leads to CSNB2-like phenotype (10).…”