Photoreceptor calcium channels: Insight from night blindness

Abstract

The genetic locus for incomplete congenital stationary night blindness (CSNB2) has been identified as the CACNA1f gene, encoding the alpha 1F calcium channel subunit, a member of the L-type family of calcium channels. The electroretinogram associated with CSNB2 implicates alpha 1F in synaptic transmission between retinal photoreceptors and bipolar cells. Using a recently developed monoclonal antibody to alpha 1F, we localize the channel to ribbon active zones in rod photoreceptor terminals of the mouse retina, supporting a role for alpha 1F in mediating glutamate release from rods. Detergent extraction experiments indicate that alpha 1F is part of a detergent-resistant active zone complex, which also includes the synaptic ribbons. Comparison of native mouse rod calcium currents with recombinant alpha 1F currents reveals that the current-voltage relationship for the native current is shifted approximately 30 mV to more hyperpolarized potentials than for the recombinant alpha 1F current, suggesting modulation of the native channel by intracellular factors. Lastly, we present evidence for L-type alpha 1D calcium channel subunits in cone terminals of the mouse retina. The presence of alpha 1D channels in cones may explain the residual visual abilities of individuals with CSNB2.

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

“…In this manuscript, however, affected mice will continue to be referred to simply as nob2, regardless of their gender (i.e., Cacna1f nob2 /Cacna1f nob2 or Cacna1f nob2 /Y). To examine whether the α 1F subunit protein was expressed in the nob2 retina, we reacted retinal cross sections with an antibody against the C-terminal 177 amino acids of rat α 1F , which recognizes α 1F in the OPL and inner plexiform layer (IPL) of WT mice (Morgans et al, 2005). As can be seen in Fig.…”

“…In the outer retina, the α 1F subunit of the VDCC is expressed in the OPL, and it has been localized specifically to both rod and cone photoreceptor terminals (Morgans et al, 2005). These data, and the fact that the α 1F subunit is critical to producing a VDCC on photoreceptor terminals that does not inactivate during sustained depolarization (Taylor & Morgans, 1998), indicate that this subunit may be the primary VDCC α 1 subunit involved in mediating tonic glutamate release from rods and cones.…”

“…This solution was replaced and sections were incubated with primary antibody diluted in blocking solution for either 1-2 h at room temperature or overnight at 4°C. Primary antibodies and dilutions were as follows: monoclonal anti-α 1F (4G2; Morgans et al, 2005, at 1:2); anti-Ctbp2/Ribeye (BD Biosciences Pharmingen, San Diego, CA, at 1:5000); anti-Protein Kinase Cα (PKC; Sigma, St Louis, MO, at 1:5000); anti-calbindin D-28K (Chemicon, Temecula, CA, at 1:1000); anti-Neurokinin-3 receptor (NK3R; Grady et al, 1996, at 1:500); rhodamine-labeled PNA (Vector Labs, Burlingame, CA, at 1 μg/ml); and anti-mGluR6 (Neuromics, Minneapolis, MN, at 1:1000). After washing in PB three times for 20 min, sections were incubated in fluorescent secondary antibodies at room temperature for 1 h. Secondary antibodies were: 1:500 dilution of CY3 conjugated goat anti-mouse (Jackson ImmunoResearch, West Grove, PA) for α 1F ; 1:1000 dilution of Alexa 488 conjugated goat anti-mouse (Ribeye); and 1:1000 dilution of Alexa 488 or Alexa 568 goat anti-rabbit (mGluR6, PKC, calbindin, NK3R; Invitrogen Life Technologies, Carlsbad, CA).…”

“…Expression studies in heterologous systems show that α 1F combines with β and α 2 /δ subunits to form functional L-type VDCCs (Koschak et al, 2003;Baumann et al, 2004;McRory et al, 2004;Hoda et al, 2005;Morgans et al, 2005). In the outer retina, the α 1F subunit of the VDCC is expressed in the OPL, and it has been localized specifically to both rod and cone photoreceptor terminals (Morgans et al, 2005).…”

The expression of L‐type voltage‐gated calcium channels in retinal photoreceptors is under circadian control

“…In both mice (Morgans et al 2005) and rats (Xu et al 2002), VGCCa1D is distributed in the cell bodies and terminals of photoreceptors, while VGCCa1C is not observed in rat photoreceptors. We found that in dissociated chick retina cultures, VGCCa1C appeared on the membrane boundary and processes of almost all cell types including photoreceptors, glia cells, and fibroblasts, while VGCCa1D was present mainly in the inner segment of photoreceptors as well as other retinal neurons but not in glia cells or fibroblasts.…”