Characterization of C-terminal Splice Variants of Cav1.4 Ca2+ Channels in Human Retina

Abstract

Voltage-gated Ca(2+) channels (Cav) undergo extensive alternative splicing that greatly enhances their functional diversity in excitable cells. Here, we characterized novel splice variants of the cytoplasmic C-terminal domain of Cav1.4 Ca(2+) channels that regulate neurotransmitter release in photoreceptors in the retina. These variants lack a portion of exon 45 and/or the entire exon 47 (Cav1.4Δex p45, Cav1.4Δex 47, Cav1.4Δex p45,47) and are expressed in the retina of primates but not mice. Although the electrophysiological properties of Cav1.4Δex p45 are similar to those of full-length channels (Cav1.4FL), skipping of exon 47 dramatically alters Cav1.4 function. Deletion of exon 47 removes part of a C-terminal automodulatory domain (CTM) previously shown to suppress Ca(2+)-dependent inactivation (CDI) and to cause a positive shift in the voltage dependence of channel activation. Exon 47 is crucial for these effects of the CTM because variants lacking this exon show intense CDI and activate at more hyperpolarized voltages than Cav1.4FL The robust CDI of Cav1.4Δex 47 is suppressed by CaBP4, a regulator of Cav1.4 channels in photoreceptors. Although CaBP4 enhances activation of Cav1.4FL, Cav1.4Δex 47 shows similar voltage-dependent activation in the presence and absence of CaBP4. We conclude that exon 47 encodes structural determinants that regulate CDI and voltage-dependent activation of Cav1.4, and is necessary for modulation of channel activation by CaBP4.

Retinoschisin Facilitates the Function of L-Type Voltage-Gated Calcium Channels

“…When co-transfected with the full length Cav1.4 and the axillary subunits (Cav1.4 + β2 + α2δ1), RS1 significantly enhanced the Cav1.4-LTCC current density (Figure 3 ) and the voltage-dependent activation (Figure 3B ). Unlike Cav1.2- or Cav1.3-LTCCs, full length Cav1.4 without a deletion of exon 47 shows no discernable CDI (Baumann et al, 2004 ) and displays unusually slow voltage-dependent inactivation (McRory et al, 2004 ; Haeseleer et al, 2016 ). Co-transfection with RS1 and functional Cav1.4-LTCC did not alter the CDI property of Cav1.4-LTCCs (Figure 3C ).…”

Circadian regulation in the retina: From molecules to network

“…L‐type calcium channels (LTCCs) mediate a voltage‐dependent and depolarization‐induced calcium influx and regulate diverse biological processes such as contraction, secretion, neurotransmission, differentiation, and gene expression in many different cell types (Barnes & Kelly, ; Benitah et al., ; Catterall & Few, ; Catterall, Perez‐Reyes, Snutch, & Striessnig, ; Dolphin, , ; Joiner & Lee, ; Thorneloe & Nelson, ). There are three major LTCCs expressed in the vertebrate retina: Cav1.2, Cav1.3, and Cav1.4 (Ahlijanian, Westenbroek, & Catterall, ; Barnes & Kelly, ; Cristofanilli, Mizuno, & Akopian, ; Firth, Morgan, Boelen, & Morgans, ; Haeseleer, Williams, & Lee, ; Haeseleer et al., ; Kersten et al., ; Knoflach et al., ; Ko et al., ; Lee et al., ; Liu et al., ; Mizuno, Barabas, Krizaj, & Akopian, ; Morgans, ; Morgans et al., ; Shi, Chang, Yu, Ko, & Ko, ; Strom et al., ; Wu, Marmorstein, Striessnig, & Peachey, ; Xing, Akopian, & Krizaj, ; Xu, Zhao, & Yang, ; Zou, Lee, & Yang, ). In photoreceptors, Cav1.4 is exclusively expressed at the synaptic terminals and responsible for the tonic release of glutamate in the dark as a result of depolarization‐evoked activation of LTCCs (Haeseleer et al., , ; Knoflach et al., ; Morgans, ).…”

“…There are three major LTCCs expressed in the vertebrate retina: Cav1.2, Cav1.3, and Cav1.4 (Ahlijanian, Westenbroek, & Catterall, ; Barnes & Kelly, ; Cristofanilli, Mizuno, & Akopian, ; Firth, Morgan, Boelen, & Morgans, ; Haeseleer, Williams, & Lee, ; Haeseleer et al., ; Kersten et al., ; Knoflach et al., ; Ko et al., ; Lee et al., ; Liu et al., ; Mizuno, Barabas, Krizaj, & Akopian, ; Morgans, ; Morgans et al., ; Shi, Chang, Yu, Ko, & Ko, ; Strom et al., ; Wu, Marmorstein, Striessnig, & Peachey, ; Xing, Akopian, & Krizaj, ; Xu, Zhao, & Yang, ; Zou, Lee, & Yang, ). In photoreceptors, Cav1.4 is exclusively expressed at the synaptic terminals and responsible for the tonic release of glutamate in the dark as a result of depolarization‐evoked activation of LTCCs (Haeseleer et al., , ; Knoflach et al., ; Morgans, ). Cav1.3 is also detected at the synaptic terminal (Firth et al., ; Shi, Chiang, et al., ), but it might not be as essential for mediating neurotransmitter release as Cav1.4 (Shi, Ko, & Ko, ; Shi, Chang, et al., ).…”

Channeling Vision: CaV1.4—A Critical Link in Retinal Signal Transmission

“…While heterologous systems are valuable for determining basic kinetic properties of Ca V 1.4, none of them has yet captured fully the complexities of the photoreceptor calcium current. In vivo, retinal photoreceptors express simultaneously several splice isoforms of Ca V 1.4 with different kinetics [ 48 , 49 ] and associate with unique modulatory subunits (discussed below: Ca V 1.4 Subunit Composition ) [ 17 ]. Additional factors, including physiological temperature (37°C) [ 50 ], calmodulin (CaM) coexpression [ 51 – 53 ], and calcium-binding protein 4 (CaBP4) coexpression [ 54 , 55 ], also significantly modulate biophysical parameters of the channel.…”

“…Over 20 splice isoforms of CACNA1F have been identified, several of which have significantly disparate dynamics [ 48 , 49 ]. Most remarkably, several splice variants exist in which the C-terminus is truncated or otherwise disrupted.…”