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Dysregulation of Ca(v)1.4 channels disrupts the maturation of photoreceptor synaptic ribbons in congenital stationary night blindness type 2.

Liu X, Kerov V, Haeseleer F, Majumder A, Artemyev N, Baker SA, Lee A - Channels (Austin) (2013)

Bottom Line: Using Cav 1.4-selective antibodies, we found that Cav 1.4 channels associate with ribbon precursors early in development and are concentrated at both rod and cone PR synapses in the mature retina.However, after postnatal day 13, many PR ribbons retain the immature morphology.Our results demonstrate the importance of proper Cav 1.4 function for efficient PR synapse maturation, and that dysregulation of Cav 1.4 channels in CSNB2 may have synaptopathic consequences.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular Physiology and Biophysics; University of Iowa; Iowa City, IA USA; Departments of Otolaryngology-Head and Neck Surgery, and Neurology; University of Iowa; Iowa City, IA USA.

ABSTRACT
Mutations in the gene encoding Cav 1.4, CACNA1F, are associated with visual disorders including X-linked incomplete congenital stationary night blindness type 2 (CSNB2). In mice lacking Cav 1.4 channels, there are defects in the development of "ribbon" synapses formed between photoreceptors (PRs) and second-order neurons. However, many CSNB2 mutations disrupt the function rather than expression of Cav 1.4 channels. Whether defects in PR synapse development due to altered Cav 1.4 function are common features contributing to the pathogenesis of CSNB2 is unknown. To resolve this issue, we profiled changes in the subcellular distribution of Cav 1.4 channels and synapse morphology during development in wild-type (WT) mice and mouse models of CSNB2. Using Cav 1.4-selective antibodies, we found that Cav 1.4 channels associate with ribbon precursors early in development and are concentrated at both rod and cone PR synapses in the mature retina. In mouse models of CSNB2 in which the voltage-dependence of Cav 1.4 activation is either enhanced (Cav 1.4I756T) or inhibited (CaBP4 KO), the initial stages of PR synaptic ribbon formation are largely unaffected. However, after postnatal day 13, many PR ribbons retain the immature morphology. This synaptic abnormality corresponds in severity to the defect in synaptic transmission in the adult mutant mice, suggesting that lack of sufficient mature synapses contributes to vision impairment in Cav 1.4I756T and CaBP4 KO mice. Our results demonstrate the importance of proper Cav 1.4 function for efficient PR synapse maturation, and that dysregulation of Cav 1.4 channels in CSNB2 may have synaptopathic consequences.

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Figure 6. ERGs from WT and Cav1.4I756T mice. (A) Representative voltage traces from flash ERG recordings in 5–6-week old dark- adapted or light-adapted WT and Cav1.4I756T mice. Arrowheads indicate time of flash, numbers indicate flash intensities (cd•s /m2). (B and C) b-wave amplitudes (B) and a-wave amplitudes (C) measured from recordings of dark-adapted mice obtained as in (A). Points represent the mean ± SEM (n = 8; left and right eyes from 4 mice). Smooth lines represent fits from double (WT in B) or single sigmoidal functions.
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Figure 6: Figure 6. ERGs from WT and Cav1.4I756T mice. (A) Representative voltage traces from flash ERG recordings in 5–6-week old dark- adapted or light-adapted WT and Cav1.4I756T mice. Arrowheads indicate time of flash, numbers indicate flash intensities (cd•s /m2). (B and C) b-wave amplitudes (B) and a-wave amplitudes (C) measured from recordings of dark-adapted mice obtained as in (A). Points represent the mean ± SEM (n = 8; left and right eyes from 4 mice). Smooth lines represent fits from double (WT in B) or single sigmoidal functions.

Mentions: To determine if the impaired ribbon maturation in the mutant mice corresponded to deficits in PR synaptic transmission, we performed ERG analyses, which monitor light-induced changes in the electrical activity of presynaptic PRs (a-wave) and postsynaptic second-order neurons (b-waves) (Fig. 6A). Previous ERG analyses established strongly reduced b-waves in CaBP4 KO mice under dark-adapted conditions to measure rod PR transmission.19 Therefore, we restricted analyses to comparisons of Cav1.4I756T mice with WT mice. Due to the presence of rod- and cone-driven responses in WT mice, plots of b-wave amplitudes against flash intensity were fit with a double-sigmoidal function (Fig. 6B). In contrast, the corresponding data in Cav1.4I756T mice were described by a single sigmoidal fit (Fig. 6B). Since ERG recordings under light-adapted conditions indicated that Cav1.4I756T mice lack cone-driven responses (Fig. 6A), we assumed that the b-wave responses from dark-adapted Cav1.4I756T mice are largely due to rod transmission. Compared with rod-driven responses in WT mice, b-waves were significantly reduced, although not abolished, in Cav1.4I756T mice (~66–74% compared with WT, at light intensities (I) < -1.0 log cd•s/m2, p < 0.001 by ANOVA; Fig. 6B). In addition, the sensitivity of rod-rod bipolar synaptic transmission in Cav1.4I756T mice (I1/2) was significantly reduced (I1/2 = -3.2 ± 0.2 log cd•s /m2 for WT vs. -0.3 ± 0.6 log cd s /m2 for I756T; p < 0.001 by t-test). While the maximal a-wave amplitudes appeared to be reduced in Cav1.4I756T, the difference with WT was not significant (385.3 ± 14.2 for WT vs. 301.3 ± 48.2 for Cav1.4I756T; p = 0.1 by t-test, Fig. 6C). There was also no significant difference in the flash intensities evoking the half-maximal a-wave response (I1/2 = -0.9 ± 0.1 for WT vs. -0.5 ± 0.4 log cd•s/m2 for Cav1.4I756T; p = 0.2 by t-test). These results argued against the possibility that deficient b-wave responses in dark-adapted Cav1.4I756T mice were due to decreased sensitivity or changes in the numbers of functional rod PRs. Moreover, at light intensities at which the a-waves were nearly identical in dark-adapted WT and Cav1.4I756T mice (e.g., I < -1 log cd•s/m2), the b-wave was still significantly smaller in the latter (p < 0.001 by ANOVA; Fig. 6B and C). Taken together, these results strongly supported a major defect in PR transmission in Cav1.4I756T mice. The less severe rod-driven responses in Cav1.4I756T mice compared with CaBP4 KO and Cav1.4 KO mice7,19 are consistent with the more moderate abnormalities in ribbon maturation in the former compared with the latter 2 mice (Figs. 4and5). Our results support a requirement for normal Cav1.4 function in the maturation of rod PR synaptic ribbons, which may impact the efficacy of synaptic transmission in the adult mice.


Dysregulation of Ca(v)1.4 channels disrupts the maturation of photoreceptor synaptic ribbons in congenital stationary night blindness type 2.

Liu X, Kerov V, Haeseleer F, Majumder A, Artemyev N, Baker SA, Lee A - Channels (Austin) (2013)

Figure 6. ERGs from WT and Cav1.4I756T mice. (A) Representative voltage traces from flash ERG recordings in 5–6-week old dark- adapted or light-adapted WT and Cav1.4I756T mice. Arrowheads indicate time of flash, numbers indicate flash intensities (cd•s /m2). (B and C) b-wave amplitudes (B) and a-wave amplitudes (C) measured from recordings of dark-adapted mice obtained as in (A). Points represent the mean ± SEM (n = 8; left and right eyes from 4 mice). Smooth lines represent fits from double (WT in B) or single sigmoidal functions.
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Related In: Results  -  Collection

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Figure 6: Figure 6. ERGs from WT and Cav1.4I756T mice. (A) Representative voltage traces from flash ERG recordings in 5–6-week old dark- adapted or light-adapted WT and Cav1.4I756T mice. Arrowheads indicate time of flash, numbers indicate flash intensities (cd•s /m2). (B and C) b-wave amplitudes (B) and a-wave amplitudes (C) measured from recordings of dark-adapted mice obtained as in (A). Points represent the mean ± SEM (n = 8; left and right eyes from 4 mice). Smooth lines represent fits from double (WT in B) or single sigmoidal functions.
Mentions: To determine if the impaired ribbon maturation in the mutant mice corresponded to deficits in PR synaptic transmission, we performed ERG analyses, which monitor light-induced changes in the electrical activity of presynaptic PRs (a-wave) and postsynaptic second-order neurons (b-waves) (Fig. 6A). Previous ERG analyses established strongly reduced b-waves in CaBP4 KO mice under dark-adapted conditions to measure rod PR transmission.19 Therefore, we restricted analyses to comparisons of Cav1.4I756T mice with WT mice. Due to the presence of rod- and cone-driven responses in WT mice, plots of b-wave amplitudes against flash intensity were fit with a double-sigmoidal function (Fig. 6B). In contrast, the corresponding data in Cav1.4I756T mice were described by a single sigmoidal fit (Fig. 6B). Since ERG recordings under light-adapted conditions indicated that Cav1.4I756T mice lack cone-driven responses (Fig. 6A), we assumed that the b-wave responses from dark-adapted Cav1.4I756T mice are largely due to rod transmission. Compared with rod-driven responses in WT mice, b-waves were significantly reduced, although not abolished, in Cav1.4I756T mice (~66–74% compared with WT, at light intensities (I) < -1.0 log cd•s/m2, p < 0.001 by ANOVA; Fig. 6B). In addition, the sensitivity of rod-rod bipolar synaptic transmission in Cav1.4I756T mice (I1/2) was significantly reduced (I1/2 = -3.2 ± 0.2 log cd•s /m2 for WT vs. -0.3 ± 0.6 log cd s /m2 for I756T; p < 0.001 by t-test). While the maximal a-wave amplitudes appeared to be reduced in Cav1.4I756T, the difference with WT was not significant (385.3 ± 14.2 for WT vs. 301.3 ± 48.2 for Cav1.4I756T; p = 0.1 by t-test, Fig. 6C). There was also no significant difference in the flash intensities evoking the half-maximal a-wave response (I1/2 = -0.9 ± 0.1 for WT vs. -0.5 ± 0.4 log cd•s/m2 for Cav1.4I756T; p = 0.2 by t-test). These results argued against the possibility that deficient b-wave responses in dark-adapted Cav1.4I756T mice were due to decreased sensitivity or changes in the numbers of functional rod PRs. Moreover, at light intensities at which the a-waves were nearly identical in dark-adapted WT and Cav1.4I756T mice (e.g., I < -1 log cd•s/m2), the b-wave was still significantly smaller in the latter (p < 0.001 by ANOVA; Fig. 6B and C). Taken together, these results strongly supported a major defect in PR transmission in Cav1.4I756T mice. The less severe rod-driven responses in Cav1.4I756T mice compared with CaBP4 KO and Cav1.4 KO mice7,19 are consistent with the more moderate abnormalities in ribbon maturation in the former compared with the latter 2 mice (Figs. 4and5). Our results support a requirement for normal Cav1.4 function in the maturation of rod PR synaptic ribbons, which may impact the efficacy of synaptic transmission in the adult mice.

Bottom Line: Using Cav 1.4-selective antibodies, we found that Cav 1.4 channels associate with ribbon precursors early in development and are concentrated at both rod and cone PR synapses in the mature retina.However, after postnatal day 13, many PR ribbons retain the immature morphology.Our results demonstrate the importance of proper Cav 1.4 function for efficient PR synapse maturation, and that dysregulation of Cav 1.4 channels in CSNB2 may have synaptopathic consequences.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular Physiology and Biophysics; University of Iowa; Iowa City, IA USA; Departments of Otolaryngology-Head and Neck Surgery, and Neurology; University of Iowa; Iowa City, IA USA.

ABSTRACT
Mutations in the gene encoding Cav 1.4, CACNA1F, are associated with visual disorders including X-linked incomplete congenital stationary night blindness type 2 (CSNB2). In mice lacking Cav 1.4 channels, there are defects in the development of "ribbon" synapses formed between photoreceptors (PRs) and second-order neurons. However, many CSNB2 mutations disrupt the function rather than expression of Cav 1.4 channels. Whether defects in PR synapse development due to altered Cav 1.4 function are common features contributing to the pathogenesis of CSNB2 is unknown. To resolve this issue, we profiled changes in the subcellular distribution of Cav 1.4 channels and synapse morphology during development in wild-type (WT) mice and mouse models of CSNB2. Using Cav 1.4-selective antibodies, we found that Cav 1.4 channels associate with ribbon precursors early in development and are concentrated at both rod and cone PR synapses in the mature retina. In mouse models of CSNB2 in which the voltage-dependence of Cav 1.4 activation is either enhanced (Cav 1.4I756T) or inhibited (CaBP4 KO), the initial stages of PR synaptic ribbon formation are largely unaffected. However, after postnatal day 13, many PR ribbons retain the immature morphology. This synaptic abnormality corresponds in severity to the defect in synaptic transmission in the adult mutant mice, suggesting that lack of sufficient mature synapses contributes to vision impairment in Cav 1.4I756T and CaBP4 KO mice. Our results demonstrate the importance of proper Cav 1.4 function for efficient PR synapse maturation, and that dysregulation of Cav 1.4 channels in CSNB2 may have synaptopathic consequences.

Show MeSH
Related in: MedlinePlus