Background: Mutations in CACNA1F, which encodes the Cav1.4 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: Cacna1fG305X: Adults were characterized by visual function (photopic optokinetic response, OKR); gene expression (microarray) and by cell death (TUNEL) and synaptic development (TEM). Cacna1fnob2: 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 Cacna1fG305Xmice caused loss of cone cell ribbons, failure of OPL synaptogenesis, ERG b-wave and absence of OKR. In Cacna1fnob2 mice alternative ETn splicing produced ~90% Cacna1f mRNA having a stop codon, but ~10% mRNA encoding a complete polypeptide. Cacna1fnob2 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: Cav1.4 plays a key role in photoreceptor synaptogenesis and synaptic function in mouse retina. Cacna1fG305X is a true knockout model for human CSNB2, with prominent defects in cone and rod function. Cacna1fnob2 is an incomplete knockout model for CSNB2, because alternative splicing in an ETn element leads to some full-length Cav1.4 protein, and some cones surviving to drive photopic visual responses.
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Supported by: FFB-Canada, CIHR, NSERC, and AHFMR.
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