Localization of Rod Bipolar Cells in the Mammalian Retina Using an Antibody Against the α₁c L-type Ca²⁺ Channel

Abstract

Bipolar cells transmit stimuli via graded changes in membrane potential and neurotransmitter release is modulated by Ca²⁺ influx through L-type Ca²⁺ channels. The purpose of this study was to determine whether the α₁c subunit of L-type voltage-gated Ca²⁺ channel (α₁c L-type Ca²⁺ channel) colocalizes with protein kinase C alpha (PKC-α), which labels rod bipolar cells. Retinal whole mounts and vertical sections from mouse, hamster, rabbit, and dog were immunolabeled with antibodies against PKC-α and α₁c L-type Ca²⁺ channel, using fluorescein isothiocyanate (FITC) and Cy5 as visualizing agents. PKC-α-immunoreactive cells were morphologically identical to rod bipolar cells as previously reported. Their cell bodies were located within the inner nuclear layer, dendritic processes branched into the outer plexiform layer, and axons extended into the inner plexiform layer. Immunostaining showed that α₁c L-type Ca²⁺ channel colocalized with PKC-α in rod bipolar cells. The identical expression of PKC-α and α₁c L-type Ca²⁺ channel indicates that the α₁c L-type Ca²⁺ channel has a specific role in rod bipolar cells, and the antibody against the α₁c L-type Ca²⁺ channel may be a useful marker for studying the distribution of rod bipolar cells in mouse, hamster, rabbit, and dog retinas.

I.  Introduction

Bipolar cells are glutamatergic second-order neurons that transmit signals from photoreceptors to amacrine and ganglion cells in the inner plexiform layer (IPL) of the retina. According to the source of presynaptic input, bipolar cells are classified into two types: rod and cone bipolar cells. According to the current concept, rod photoreceptors connect only to rod bipolar cells, and cone photoreceptors connect only to cone bipolar cells. Multiple subtypes of cone bipolar cells have been reported, but there is only one type of rod bipolar cell [6].

Various cellular processes are controlled by Ca²⁺ concentration, and neurotransmitter release is related to the influx of Ca²⁺ at presynaptic terminals [12, 17, 26]. Thus, the voltage-gated Ca²⁺ channels play an important role in bipolar cell synaptic transmission. Two types of Ca²⁺ currents have been measured in mouse bipolar cells using patch-clamp recordings: transient (T-) and long lasting (L-) types [5, 21]. The L-type Ca²⁺ channel is composed of four subunits (α₁, α₂δ, β, and γ), shows pharmacological sensitivity to dihydropyridines (DHP), and requires relatively strong depolarization for activation. The α₁ subunit is a transmembrane protein that forms the ion channel containing the DHP-binding site [19]. Among the four subunits of the L-type Ca²⁺ channels, α₁ is the key subunit that modulates voltage-gated Ca²⁺ channel activity. There are several α₁ subunits [8], and an antibody against the α₁c subunit of L-type voltage-gated Ca²⁺ channel (α₁c L-type Ca²⁺ channel) was used in the present study.

Protein kinase C (PKC) is an important secondary messenger in the nervous system. It plays a fundamental role in the control of cell proliferation, differentiation, gene expression, and modulation of the sensitivity of channels and receptors. Enzymatic and cloning studies have revealed that PKC exists as a family of isoforms that are closely related in structure, size, and mechanism of activation. Eleven PKC isoforms have been identified, and these can be subdivided into three major groups based on their activation conditions. PKC isoforms are classified as conventional (α, β_I, β_II, γ), novel (δ, ε, η, θ), and atypical (ζ, λ) types [13]. Immunocytochemical studies indicate that conventional PKC isoforms are found in the central nervous system [9, 22]. Negishi and coworkers published the study of PKC alpha (PKC-α) localization in the retina and demonstrated intense PKC-α-immunoreactivity in rod bipolar cells [18]. PKC-α is now a commonly used marker to identify rod bipolar cells in the vertebrate retina [3, 4, 15, 16, 27].

The purpose of this study was to investigate whether the α₁c L-type Ca²⁺ channel localizes to a specific cell type in the mammalian retina. Our results show that cells labeled with an antibody against α₁c L-type Ca²⁺ channel exhibited the morphology of rod bipolar cells with PKC-α immunoreactivity. This study indicates that α₁c L-type Ca²⁺ channel may be another marker for rod bipolar cells in the mammalian retina.

II.  Materials and Methods

Animals and tissue preparation

Four different mammalian species were used in the present study: adult C57BL/6 mice (20–25 g), golden hamsters (25–30 g), New Zealand white rabbits (2.5–3.0 kg), and Korean mixed-breed dogs (4.0–5.0 kg). Animals were anesthetized with a mixture of ketamine hydrochloride (30–40 mg/kg) and xylazine (3–6 mg/kg). Proparacaine HCl (200–300 μl) was applied to the cornea to suppress blink reflexes. Immediately after eye enucleation, the retinas were isolated from the eyecup. The isolated retinas were fixed for 1–2 hr in a fixative solution containing 4% paraformaldehyde in 0.1 M phosphate buffer (PB, pH 7.4) with 0.002% CaCl₂ added. The experiments were conducted in accordance with the Association for Research in Vision and Ophthalmology (ARVO) guidelines for the use of animals with the approval of the committee of Kyungpook National University.

Immunocytochemical procedure

The retinal tissues were processed as whole mounts or 50 μm vertical sections, which were cut using a Vibratome 3000 Plus Sectioning System (St. Louis, Missouri, USA). The immunocytochemical methods have been described in detail previously [14]. The primary antibodies used were monoclonal anti-PKC (1:100, Amersham, Arlington Heights, Illinois, USA) and polyclonal anti-α₁c L-type voltage-gated Ca²⁺ channel (1:200–1000, Chemicon, Temecula, California, USA). Secondary antibodies used were fluorescein isothiocyanate (FITC)-conjugated horse anti-mouse IgG (1:50–250, Vector Laboratories, Burlingame, California, USA) and Cy5-conjugated goat anti-rabbit IgG (1:50, Jackson ImmunoResearch, West Grove, Pennsylvania, USA). Following the immunocytochemical procedures, the tissues were mounted in Vectashield mounting medium (Vector Laboratories).

Characterization of antibodies

(1) A monoclonal antibody against PKC has been found to react with the α and β forms, but not the γ form. We checked its cross-reactivity using an antibody against PKC-α (1:100, #MA1-157, Thermo Fisher Scientific, Rockford, Illinois, USA), which recognizes only the α form. We found no difference between the immunostaining patterns using antibodies against PKC or PKC-α. Besides, many previous studies proved that the antibody used in this study is a specific marker of rod bipolar cells [6, 9, 18].

(2) The information provided with the polyclonal antibody against the α₁c L-type Ca²⁺ channel states that the antibody does not cross-react with any other Ca²⁺ channel antigens tested. To demonstrate the specificity of the antibody against the α₁c L-type Ca²⁺ channel, we performed a preabsorption control test in the retina of four species. To inactivate the antibody, blocking peptide of the anti-α₁c L-type Ca²⁺ channel (#sc-16229-P, Santa Cruz Biotechnology, Santa Cruz, California, USA) was mixed with the primary antibody at 10:1 ratio and the mixture was pre-incubated for 1 hr at room temperature. Tissue sections were incubated in the preabsorbed antibody in place of the primary antibody. We confirmed that no immunoreactivity was visible in retinal cells (Fig. 1).

Fig. 1. 

Photomicrographs of the distribution and control of α₁c L-type Ca²⁺ channel-immunoreactive cells in the retina. (A, D, G, J) Anti-α₁c L-type Ca²⁺ channel-immunoreactive cells. (B, E, H, K) Sections before preabsorption. (C, F, I, L) The α₁c L-type Ca²⁺ channel antibody was preabsorbed with antigen prior to tissue incubation. Preabsorption control test of antibody against the α₁c L-type Ca²⁺ channel showed no immunoreactive cells in four different mammalian retinas. INL, inner nuclear layer; IPL, inner plexiform layer. Bar=20 μm.

Data analysis

A differential interference contrast (DIC) microscopy was used to clear the immunostaining area. All labeled tissues were imaged using a laser scanning confocal microscope (LSM 700, Carl Zeiss Meditec Incorporation, Jena, Germany). Images were adjusted for brightness and contrast using Adobe Photoshop CS (Adobe Systems, Mountain View, California, USA).

III.  Results

Figure 2 shows PKC-α- and α₁c L-type Ca²⁺ channel-immunoreactive cells in vertical sections of the retina from mouse (Fig. 2A–D), hamster (Fig. 2E–H), rabbit (Fig. 2I–L), and dog (Fig. 2M–P). Antibodies against PKC-α and α₁c L-type Ca²⁺ channel labeled processes in the retina, which is consistent with the morphology of rod bipolar cells. In Figures 2A, 2E, 2I, and 2M, the DIC images were shown to clarify the layers of retina. Immunoreactive cell bodies were visible in the outermost region of the inner nuclear layer (INL), with branched dendritic processes in the outer plexiform layer (OPL). The cells also had long and thick axonal processes in the innermost region of the IPL, close to the ganglion cell layer (GCL). In Figures 2D, 2H, 2L, and 2P, the superimposed images of PKC-α- and α₁c L-type Ca²⁺ channel-immunoreactive cells demonstrate the extent of colocalization. All the rod bipolar cells stained with PKC-α were also labeled for α₁c L-type Ca²⁺ channel.

Fig. 2. 

Confocal micrographs of vertical sections from the mouse, hamster, rabbit, and dog retinas. (A, E, I, M) DIC images were used to reveal the layers of retina. (B, F, J, N) Retinal sections were immunostained for PKC-α. (C, G, K, O) Retinal sections were immunostained for α₁c L-type Ca²⁺ channel. In both labelings, the immunoreactive cells displayed the morphology of rod bipolar cells as their cell bodies in the INL extended long axonal process throughout the IPL. (D, H, L, P) Superimposed images of PKC-α and α₁c L-type Ca²⁺ channel immunolabeling revealed an almost complete of colocalization. DIC, differential interference contrast; INL, inner nuclear layer; IPL, inner plexiform layer. Bar=20 μm.

Figure 3 shows the immunoreactivity of PKC-α and α₁c L-type Ca²⁺ channel on the retinal whole mounts. The labeled cells were imaged focusing on the cell bodies of rod bipolar cells in the INL, and the images show the rod bipolar cell body mosaic in the retina of each species. Figures 3C, 3F, 3I, and 3L show that PKC-α-immunoreactive cells colocalize with α₁c L-type Ca²⁺ channel-labeling in mouse, hamster, rabbit, and dog. These results confirm that the rod bipolar cells labeled for PKC-α are also immunoreactive for α₁c L-type Ca²⁺ channel.

Fig. 3. 

Confocal micrographs of whole mounts from the mouse, hamster, rabbit, and dog retinas. (A, D, G, J) Whole mounts were immunostained for PKC-α. (B, E, H, K) Whole mounts were immunostained for α₁c L-type Ca²⁺ channel. Immunolabeling was visible within the cytoplasm of rod bipolar cells. The immunoreactive cells in the mouse and hamster formed a tight mosaic compared to those in the rabbit and dog. (C, F, I, L) Superimposed images of PKC-α and α₁c L-type Ca²⁺ channel immunostaining revealed that these proteins were colocalized. Bar=20 μm.

IV.  Discussion

The present study has revealed one important feature of the expression of α₁c L-type Ca²⁺ channel in mouse, hamster, rabbit, and dog retinas: the expression of PKC-α and α₁c L-type Ca²⁺ channel is identical and localized exclusively to rod bipolar cells in mouse, hamster, rabbit, and dog. These results show that an antibody against the α₁c L-type Ca²⁺ channel can be used as a novel marker for rod bipolar cells.

In early experimental studies investigating the expression of PKC in the retina, PKC-α was found to be expressed in cells with a morphology corresponding to rod bipolar cells [9, 18]. Since then, PKC-α has been used as a specific marker for rod bipolar cells in the retina. Our results for PKC-α expression in mouse, hamster, rabbit, and dog retinas are consistent with earlier results in other vertebrates, including cat, chicken, goldfish, guinea pig, human, rabbit, rat, and zebrafish [3, 9, 15, 16, 18, 27]. In the human retina, the antibody against PKC-α revealed the morphology of rod bipolar cells. However, besides rod bipolar cells, another cell type was also weakly immunoreactive in the retina. These were the cone bipolar cells, which stratify in the IPL [10, 15]. In the rat retina, PKC-α was not exclusively expressed in rod bipolar cells, but in some amacrine cells as well [18]. In addition, a class of amacrine cells were PKC-α-immunoreactive in the retina of catfish, turtle, and frog. However, rod bipolar cells were not labeled for PKC-α. Although many previous studies had been reported that PKC-α was expressed in rod bipolar cells in vertebrates, the reasons for the specific localization in rod bipolar cells and the different expression pattern of PKC-α among species have not been investigated yet. It is only possible to presume that these might be related to species variation. However, PKC-α is still used as a specific marker of rod bipolar cells.

The L-type Ca²⁺ channel is a type of voltage-dependent Ca²⁺ channels whose activation result in the excitation of neurons or release of neurotransmitters [1, 11, 12, 17]. In the synaptic transmission of retinal cells, Ca²⁺ influx through L-type Ca²⁺ channel is particularly important. A previous study reported an attempt to localize L-type Ca²⁺ channels in the chicken retina. In that study, antibodies against several L-type Ca²⁺ channel α₁-subunits (α₁c, α₁d, and α₁f) were used, and each L-type Ca²⁺ channel α₁-subunit showed a specific localization pattern in the retinal cells. The α₁c-subunit was expressed in Müller cells, and the α₁d-subunit labeled the cell bodies of most retinal cells. The α₁f-subunit was present in the photoreceptor terminals [7].

Our results clearly show that α₁c L-type Ca²⁺ channel-immunoreactive cells are rod bipolar cells: their cell bodies were located in the INL, dendritic processes branched into the OPL, and axons extended into the innermost region of the IPL. Although our results differ from the results shown previously in the chicken retina, we clearly show that an antibody against the α₁c L-type Ca²⁺ channel labels rod bipolar cells in mouse, hamster, rabbit, and dog. The differences between our results and previously reported data might be due to unidentified variations between species.

It has been reported that L-type Ca²⁺ channels mediate glutamate release [11, 23, 25]. It is intriguing that only rod bipolar cells express α₁c L-type Ca²⁺ channel, as these are one of several types of glutamate-releasing cells in the retina, including cone bipolar and ganglion cells. The reason for the selective expression of α₁c L-type Ca²⁺ channel is currently unclear. In previous study, PKC-α was shown to function in modulatory mechanism at rod–rod bipolar cell synapse [20]. PKC is activated by an increase in the concentration of diacylglycerol (DAG) or Ca²⁺. The antibody used in our study recognizes PKC isoforms that require Ca²⁺ for activation [13]. We propose that these related mechanisms may explain colocalization of PKC-α and α₁c L-type Ca²⁺ channel in rod bipolar cells. It has been reported that the α₁f L-type Ca²⁺ channel is responsible for visual transduction and Ca²⁺ entry into photoreceptors [8]. In addition, a mutation in the α₁f L-type Ca²⁺ channel is associated with incomplete X-linked congenital stationary night blindness [2, 24]. Although the physiological function of the α₁c L-type Ca²⁺ channel in the visual system is unknown, previous results suggest the possible important role for α₁c L-type Ca²⁺ channel; this needs further investigation in the future. Our results also imply the possibility of a close functional relationship between the α₁c L-type Ca²⁺ channel and PKC-α in rod bipolar cells.

In conclusion, our results demonstrate that α₁c L-type Ca²⁺ channel is expressed in rod bipolar cells in the retina of mouse, hamster, rabbit, and dog. The colocalization of PKC-α and α₁c L-type Ca²⁺ channel suggests that the α₁c L-type Ca²⁺ channel is a novel specific marker for rod bipolar cells in retina.

V.  Acknowledgments

We thank Cactus Communications for proofreading the manuscript. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by Ministry of Education (NRF-2013R1A1A2059568).

VI. References

• 1   Barnes,  S. and  Kelly,  M. E. (2002) Calcium channels at the photoreceptor synapse. Adv. Exp. Med. Biol. 514; 465–476.
• 2   Bech-Hansen,  N. T.,  Naylor,  M. J.,  Maybaum,  T. A.,  Pearce,  W. G.,  Koop,  B.,  Fishman,  G. A.,  Mets,  M.,  Musarella,  M. A. and  Boycott,  K. M. (1998) Loss-of-function mutations in a calcium-channel alpha1-subunit gene in Xp11.23 cause incomplete X-linked congenital stationary night blindness. Nat. Genet. 19; 264–267.
• 3   Caminos,  E.,  Velasco,  A.,  Jarrín,  M.,  Lillo,  C.,  Jimeno,  D.,  Aijón,  J. and  Lara,  J. M. (2000) A comparative study of protein kinase C-like immunoreactive cells in the retina. Brain Behav. Evol. 56; 330–339.
• 4   Casini,  G.,  Grassi,  A.,  Trasarti,  L. and  Bagnoli,  P. (1996) Developmental expression of protein kinase C immunoreactivity in rod bipolar cells of the rabbit retina. Vis. Neurosci. 13; 817–831.
• 5   de la Villa,  P.,  Vaquero,  C. F. and  Kaneko,  A. (1998) Two types of calcium currents of the mouse bipolar cells recorded in the retinal slice preparation. Eur. J. Neurosci. 10; 317–323.
• 6   Euler,  T. and  Wässle,  H. (1995) Immunocytochemical identification of cone bipolar cells in the rat retina. J. Comp. Neurol. 361; 461–478.
• 7   Firth,  S. I.,  Morgan,  I. G.,  Boelen,  M. K. and  Morgans,  C. W. (2001) Localization of voltage-sensitive L-type calcium channels in the chicken retina. Clin. Exp. Ophthalmol. 29; 183–187.
• 8   Furukawa,  T. (2013) Types of voltage-gated calcium channels: molecular and electrophysiological views. Curr. Hypertens. Rev. 9; 170–181.
• 9   Greferath,  U.,  Grünert,  U. and  Wässle,  H. (1990) Rod bipolar cells in the mammalian retina show protein kinase C-like immunoreactivity. J. Comp. Neurol. 301; 433–442.
• 10   Haverkamp,  S.,  Haeseleer,  F. and  Hendrickson,  A. (2003) A comparison of immunocytochemical markers to identify bipolar cell types in human and monkey retina. Vis. Neurosci. 20; 589–600.
• 11   Heidelberger,  R.,  Heinemann,  C.,  Neher,  E. and  Matthews,  G. (1994) Calcium dependence of the rate of exocytosis in a synaptic terminal. Nature 371; 513–515.
• 12   Hofmann,  F.,  Biel,  M. and  Flockerzi,  V. (1994) Molecular basis for Ca2+ channel diversity. Annu. Rev. Neurosci. 17; 399–418.
• 13   Hug,  H. and  Sarre,  T. F. (1993) Protein kinase C isoenzymes: divergence in signal transduction? Biochem. J. 291; 329–343.
• 14   Jeon,  M.-H. and  Jeon,  C.-J. (1998) Immunocytochemical localization of calretinin containing neurons in retina from rabbit, cat, and dog. Neurosci. Res. 32; 75–84.
• 15   Kolb,  H.,  Zhang,  L. and  Dekorver,  L. (1993) Differential staining of neurons in the human retina with antibodies to protein kinase C isozymes. Vis. Neurosci. 10; 341–351.
• 16   Kosaka,  J.,  Suzuki,  A.,  Morii,  E. and  Nomura,  S. (1998) Differential localization and expression of alpha and beta isoenzymes of protein kinase C in the rat retina. J. Neurosci. Res. 54; 655–663.
• 17   Llinás,  R.,  Sugimori,  M. and  Silver,  R. B. (1995) The concept of calcium concentration microdomains in synaptic transmission. Neuropharmacology 34; 1443–1451.
• 18   Negishi,  K.,  Kato,  S. and  Teranishi,  T. (1988) Dopamine cells and rod bipolar cells contain protein kinase C-like immunoreactivity in some vertebrate retinas. Neurosci. Lett. 94; 247–252.
• 19   Perez-Reyes,  E.,  Kim,  H.-S.,  Lacerda,  A. E.,  Horne,  W.,  Wei,  X. Y.,  Rampe,  D.,  Campbell,  K. P.,  Brown,  A. M. and  Birnbaumer,  L. (1989) Induction of calcium currents by the expression of the alpha 1-subunit of the dihydropyridine receptor from skeletal muscle. Nature 340; 233–236.
• 20   Rampino,  M. A. and  Nawy,  S. A. (2011) Relief of Mg²⁺-dependent inhibition of TRPM1 by PKCα at the rod bipolar cell synapse. J. Neurosci. 31; 13596–13603.
• 21   Satoh,  H.,  Aoki,  K.,  Watanabe,  S. I. and  Kaneko,  A. (1998) L-type calcium channels in the axon terminal of mouse bipolar cells. Neuroreport 9; 2161–2165.
• 22   Saito,  N.,  Kikkawa,  U.,  Nishizuka,  Y. and  Tanaka,  C. (1988) Distribution of protein kinase C-like immunoreactive neurons in rat brain. J. Neurosci. 8; 369–382.
• 23   Schmitz,  Y. and  Witkovsky,  P. (1997) Dependence of photoreceptor glutamate release on a dihydropyridine-sensitive calcium channel. Neuroscience 78; 1209–1216.
• 24   Strom,  T. M.,  Nyakatura,  G.,  Apfelstedt-Sylla,  E.,  Hellebrand,  H.,  Lorenz,  B.,  Weber,  B. H.,  Wutz,  K.,  Gutwillinger,  N.,  Rüther,  K.,  Drescher,  B.,  Sauer,  C.,  Zrenner,  E.,  Meitinger,  T.,  Rosenthal,  A. and  Meindl,  A. (1998) An L-type calcium-channel gene mutated in incomplete X-linked congenital stationary night blindness. Nat. Genet. 19; 260–263.
• 25   Tachibana,  M.,  Okada,  T.,  Arimura,  T.,  Kobayashi,  K. and  Piccolino,  M. (1993) Dihydropyridine-sensitive calcium current mediates neurotransmitter release from bipolar cells of the goldfish retina. J. Neurosci. 13; 2898–2909.
• 26   Tsien,  R. W.,  Lipscombe,  D.,  Madison,  D. V.,  Bley,  K. R. and  Fox,  A. P. (1988) Multiple types of neuronal calcium channels and their selective modulation. Trends Neurosci. 11; 431–438.
• 27   Zhang,  D. R. and  Yeh,  H. H. (1991) Protein kinase C-like immunoreactivity in rod bipolar cells of the rat retina: a developmental study. Vis. Neurosci. 6; 429–437.