Essential role of Ca2+-binding protein 4, a Cav1.4 channel regulator, in photoreceptor synaptic function

Abstract

CaBP1-8 are neuronal Ca 2+ -binding proteins with similarity to calmodulin (CaM). Here we show that CaBP4 is specifically expressed in photoreceptors, where it is localized to synaptic terminals. The outer plexiform layer, which contains the photoreceptor synapses with secondary neurons, was thinner in the Cabp4 −/− mice than in control mice. Cabp4 −/− retinas also had ectopic synapses originating from rod bipolar and horizontal cells that extended into the outer nuclear layer. Responses of Cabp4 −/− rod bipolars were reduced in sensitivity about 100-fold. Electroretinograms (ERGs) indicated a reduction in cone and rod synaptic function. The phenotype of Cabp4 −/− mice shares similarities with that of incomplete congenital stationary night blindness (CSNB2) patients. CaBP4 directly associated with the C-terminal domain of the Ca v 1.4 α 1 -subunit and shifted the activation of Ca v 1.4 to hyperpolarized voltages in transfected cells. These observations indicate that CaBP4 is important for normal synaptic function, probably through regulation of Ca 2+ influx and neurotransmitter release in photoreceptor synaptic terminals.L-type Ca 2+ channels are involved in neuronal differentiation and outgrowth and in synaptic plasticity 1,2 . At many ribbon synapses, Ca 2+ influx through L-type Ca 2+ channels triggers neurotransmitter release 3-5 . The α 1 -subunit of the L-type Ca v 1.4 channel (Ca v 1.4α1) is specific to photoreceptors and is present at highest density in the synaptic terminals 5,6 . Compared with other L-type Ca 2+ channels, Ca v 1.4 channels are activated at relatively more negative voltages and show slow inactivation 7-9 , important properties for the ability of photoreceptors to sustain continual glutamate release in the dark 4,10 . Null mutations in Ca v 1.4α1 are responsible for an X-linked disorder, CSNB2 (refs. 11 ,12 ). ERGs of these patients indicate that a deficit may occur in transmission of signals from rod photoreceptors to bipolar cells. In mice, deletion of the β 2 -subunit, another component of the photoreceptor L-type channel, alters the expression of Ca v 1.4 and produces a phenotype similar to that seen in CSNB2 patients 13 .CaBPs, a subfamily of calmodulin (CaM)-like neuronal Ca 2+ -binding proteins 14 , modulate voltage-dependent Ca 2+ channels (VDCCs) and inositol triphosphate receptors 15-17 . Here we show that CaBP4, which has only been partially characterized in silico 14 , is found specifically

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

“…These mice involve the β 2 subunit of the VDCC (Ball et al, 2002;Gregg et al, 2002) or calcium-binding protein 4 (Haeseleer et al, 2004). Similar abnormalities also have been noted in mice lacking expression of bassoon (Dick et al, 2003) as well as in mice that lack functional rod or cone photoreceptors (Claes et al, 2004).…”

“…When only synaptic transmission is compromised, the a-wave component, reflecting activity of rod and cone photoreceptors (Lamb, 1996), is spared, while the b-wave, reflecting predominantly rod or cone depolarizing bipolar cell (DBC) activity (Robson & Frishman, 1995, 1998Kofuji et al, 2000;Sharma et al, 2005), is reduced. This "negative" ERG pattern has now been reported in mice with mutations in genes encoding mGluR6 (Masu et al, 1995), nyctalopin (Pardue et al, 1998), the G-protein subunit Gα O1 (Dhingra et al, 2000(Dhingra et al, , 2002, the β 2 subunit of VDCCs (Ball et al, 2002), the presynaptic cytomatrix protein bassoon (Dick et al, 2003), or calcium-binding protein 4 (Haeseleer et al, 2004). The wide variety of proteins essential for normal synaptic transmission is suggestive of the intricacy of the synaptic complex linking photoreceptors and DBCs.…”

Synaptic transmission at retinal ribbon synapses

“…However, co-expression of Ca v 1.4 channels with a Ca 2+ -binding protein that is specifically found in photoreceptors (CaBP4) shifts the activation function to a range of membrane potentials similar to those seen in native channels (Haeseleer et al, 2004). This important finding is a cautionary note for comparisons with studies of Ca 2+ channels in expression systems in which the full complement of modulatory proteins may be lacking.…”

The Diversity of Calcium Sensor Proteins in the Regulation of Neuronal Function

“…It is expressed in the retina, where it localizes to synaptic terminals and has also been detected in auditory inner hair cells. CaBP4 modulates voltage gated Ca 2þ -channels and directly associates with the carboxyl terminus of the Ca V 1.4 a1 pore-forming subunit, shifting the activation range of the channel to more hyperpolarized voltages in transfected cells (Haeseleer et al 2004). CaBP4 has also been shown to eliminate Ca 2þ -dependent inactivation of Ca V 1.3 channels, which is likely to be important in the modulation of these channels in inner hair cells, where Ca 2þ -dependent inactivation is weak or absent, probably allowing the audition of sustained sounds (Yang et al 2006).…”

“…CaBP4 function has been convincingly linked to disease and mutations in this gene generate defects in retinal function. Knockout of CaBP4 was shown to cause a phenotype similar to that of incomplete congenital stationary night blindness patients (Haeseleer et al 2004) and mutations in CaBP4 can cause autosomal recessive night blindness (Zeitz et al 2006). Patients with mutations in the CaBP4 gene have been identified, which display congenital stationary night blindness.…”