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

Abstract

Voltage-gated Cav1 and Cav2 Ca2+ channels are comprised of a pore-forming a1 subunit (Cav1.1-1.4, Cav2.1-2.3) and auxiliary β (β1-4) and α2δ (α2δ-1-4) subunits. The properties of these channels vary with distinct combinations of Cav subunits and alternative splicing of the encoding transcripts. Therefore, the impact of disease-causing mutations affecting these channels may depend on the identities of Cav subunits and splice variants. Here, we analyzed the effects of a congenital stationary night blindness type 2 (CSNB2)-causing mutation, I745T (IT), in Cav1.4 channels typical of those in human retina: Cav1.4 splice variants with or without exon 47 (Cav1.4+ex47, Cav1.4Δex47, respectively), and the auxiliary subunits, β2x13 and α2δ-4. We find that IT caused both Cav1.4 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 β2x13 and α2δ-4 but not in those with β2A and α2δ-1. We conclude that the IT mutation stabilizes channel opening and alters ion selectivity of Cav1.4 in a manner that is strengthened by exclusion of exon 47 and inclusion of β2x13 and α2δ-4. Our results reveal complex actions of IT in modifying the properties of Cav1.4 channels, which may influence the pathological consequences of this mutation in retinal photoreceptors.

Function of cone and cone-related pathways in CaV1.4 IT mice

“…While some found that cones are spared 13 , 17 , 31 , 55 , 56 , other reported drastic changes compared to rods 14 , 27 . Still, a recent study showed that the Ca V 1.4.IT mutation can exert different functional phenotypes depending on splice variant and subunit composition 57 . Deeper knowledge about the channel composition in rods and cones will therefore be essential to elucidate the CSNB2 phenotype.…”

Calcium Channels in Retinal Function and Disease

“…For example, I745T causes Cav1.4ex47 to activate at even more negative voltages and to deactivate with slower kinetics as compared to Cav1.4+ex47. Moreover, I745T causes a breakdown in the Ca 2+ selectivity of Cav1.4ex47 but not in Cav1.4+ex47 (Williams et al 2020). Because Cav1.4ex47 is expressed in human but not in rodent retina (Haeseleer et al 2016), the I745T knock-in mouse strain (Knoflach et al 2013, Liu et al 2013b, Regus-Leidig et al 2014 might not reflect some of the pathological sequelae of the mutation in the context of Cav1.4ex47.…”

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

“…This has been studied for the Cav2.1 FHM1 variant T666M (2 residues N-terminal of the selectivity filter glutamate; Hans et al, 1999;Tottene et al, 2002). Mutations in the activation gate formed by the cytoplasmic ends of the S6 helices also affect ion selectivity and single channel conductance (e.g., Cav2.1-V714L, Cav1.4-I756T in IIS6; Figure 3; Hans et al, 1999;Tottene et al, 2002;Williams et al, 2020). A dramatic effect on ion selectivity has recently been reported for CACNA1F mutation I756T (Figure 3), which depends on C-terminal splicing and the associated β-subunit isoform (Williams et al, 2020).…”