Distribution of phosphorylated protein kinase C alpha in goldfish retinal bipolar synaptic terminals: control by state of adaptation and pharmacological treatment

Ancient origin of the rod bipolar cell pathway in the vertebrate retina

Vertebrates rely on rod photoreceptors for vision in low-light conditions. The specialized downstream circuit for rod signalling, called the primary rod pathway, is well characterized in mammals, but circuitry for rod signalling in non-mammals is largely unknown. Here we demonstrate that the mammalian primary rod pathway is conserved in zebrafish, which diverged from extant mammals ~400 million years ago. Using single-cell RNA sequencing, we identified two bipolar cell types in zebrafish that are related to mammalian rod bipolar cell (RBCs), the only bipolar type that directly carries rod signals from the outer to the inner retina in the primary rod pathway. By combining electrophysiology, histology and ultrastructural reconstruction of the zebrafish RBCs, we found that, similar to mammalian RBCs, both zebrafish RBC types connect with all rods in their dendritic territory and provide output largely onto amacrine cells. The wiring pattern of the amacrine cells postsynaptic to one RBC type is strikingly similar to that of mammalian RBCs and their amacrine partners, suggesting that the cell types and circuit design of the primary rod pathway emerged before the divergence of teleost fish and mammals. The second RBC type, which forms separate pathways, was either lost in mammals or emerged in fish.

The morphological characterization of orientation-biased displaced large-field ganglion cells in the central part of goldfish retina

The vertebrate retina has about 30 subtypes of ganglion cells. Each ganglion cell receives synaptic inputs from specific types of bipolar and amacrine cells ramifying at the same depth of the inner plexiform layer (IPL), each of which is thought to process a specific aspect of visual information. Here, we identified one type of displaced ganglion cell in the goldfish retina which had a large and elongated dendritic field. As a population, all of these ganglion cells were oriented in the horizontal axis and perpendicular to the dorsal-ventral axis of the goldfish eye in the central part of retina. This ganglion cell has previously been classified as Type 1.2. However, the circuit elements which synapse with this ganglion cell are not yet characterized. We found that this displaced ganglion cell was directly tracer-coupled only with homologous ganglion cells at sites containing Cx35/36 puncta. We further illustrated that the processes of dopaminergic neurons often terminated next to intersections between processes of ganglion cells, close to where dopamine D1 receptors were localized. Finally, we showed that Mb1 ON bipolar cells had ribbon synapses in the axonal processes passing through the IPL and made ectopic synapses with this displaced ganglion cell that stratified into stratum 1 of the IPL. These results suggest that the displaced ganglion cell may synapse with both Mb1 cells using ectopic ribbon synapses and OFF cone bipolar cells with regular ribbon synapses in the IPL to function in both scotopic and photopic light conditions.

The Effect of PKCα on the Light Response of Rod Bipolar Cells in the Mouse Retina

PURPOSE

Protein kinase C α (PKCα) is abundantly expressed in rod bipolar cells (RBCs) in the retina, yet the physiological function of PKCα in these cells is not well understood. To elucidate the role of PKCα in visual processing in the eye, we examined the effect of genetic deletion of PKCα on the ERG and on RBC light responses in the mouse.

METHODS

Immunofluorescent labeling was performed on wild-type (WT), TRPM1 knockout, and PKCα knockout (PKC-KO) retina. Scotopic and photopic ERGs were recorded from WT and PKC-KO mice. Light responses of RBCs were measured using whole-cell recordings in retinal slices from WT and PKC-KO mice.

RESULTS

Protein kinase C alpha expression in RBCs is correlated with the activity state of the cell. Rod bipolar cells dendrites are a major site of PKCα phosphorylation. Electroretinogram recordings indicated that loss of PKCα affects the scotopic b-wave, including a larger peak amplitude, longer implicit time, and broader width of the b-wave. There were no differences in the ERG a- or c-wave between PKCα KO and WT mice, indicating no measurable effect of PKCα in photoreceptors or the RPE. The photopic ERG was unaffected consistent with the lack of detectable PKCα in cone bipolar cells. Whole-cell recordings from RBCs in PKC-KO retinal slices revealed that, compared with WT, RBC light responses in the PKC-KO retina are delayed and of longer duration.

CONCLUSIONS

Protein kinase C alpha plays an important modulatory role in RBCs, regulating both the peak amplitude and temporal properties of the RBC light response in the rod visual pathway.

Relief of Mg²⁺-dependent inhibition of TRPM1 by PKCα at the rod bipolar cell synapse

In the retina, light onset hyperpolarizes photoreceptors and depolarizes ON bipolar cells at the sign inverting photoreceptor-ON bipolar cell synapse. Transmission at this synapse is mediated by a signaling cascade comprised of mGluR6, a G-protein containing G(αo), and the cation channel TRP melastatin 1 (TRPM1). This system is thought to be common to both the rod- and ON-cone-driven pathways, which control vision under scotopic and photopic conditions, respectively. In this study, we present evidence that the rod pathway is uniquely susceptible to modulation by PKCα at the rod-rod bipolar cell synapse. Decreased production of DAG (an activator of PKC) by inhibition of PIP₂ (phosphatidylinositol-4,5-bisphosphate) hydrolysis caused depression of the TRPM1 current. Conversely, addition of a DAG analog, 2-acetyl-1-oleoyl-sn-glycerol (OAG), potentiated the current in rod bipolar cells but not in ON-cone bipolar cells. The potentiating effects of OAG were absent both in mutant mice that lack PKCα expression and in wild-type mice in which enzymatic activity of PKCα was pharmacologically inhibited. In addition, we found that, like other members of the TRPM subfamily, TRPM1 current is susceptible to voltage-independent inhibition by intracellular magnesium, and that modulation by PKCα relieves this inhibition, as the potentiating effects of OAG are absent in low intracellular magnesium. We conclude that activation of PKCα initiates a modulatory mechanism at the rod-rod bipolar cell synapse whose function is to reduce inhibition of the TRPM1 current by magnesium, thereby increasing the gain of transmission at this synapse.

Prox1 expression in rod precursors and Müller cells

The transcription factor Prox1 acts in rodent retinogenesis, at least in promoting cell cycle withdrawal and horizontal cell production. In the mature retina, this protein is detected at the inner nuclear layer of all vertebrate groups. We have made a neurochemical characterisation of Prox1(+) cell types in two different vertebrate groups: mammals and fish. As well as Prox1(+) horizontal cells, we have observed Prox1(+)/PKC-alpha(+) rod bipolar cells in mouse and cone ON and mixed b bipolar cells in goldfish. In mouse, only some CB(+) and CR(+) amacrine cells are Prox1(+) and the TH(+) and CR(+) amacrine cells are Prox1(-). However, in goldfish all CR(+) amacrine cells and TH(+) interplexiform cells are Prox1(+) and in the GCL displaced amacrine cells are also Prox1(+). Besides its expression in different interneuron subpopulations, we demonstrate, for the first time, the presence of Prox1 in the GS(+) and CRALBP(+) Müller cells in the retina of adult mammals and in developing and mature retina of fish. The presence of Prox1 in these cells appears to be related to survival or maintenance of their phenotype. We also demonstrate that in fish, where retinal formation persists into adulthood, Prox1 is expressed in dividing PCNA(+) cells at the peripheral growing zone, in rod progenitors at the inner and outer nuclear layers as well as in early progenitors during a retinal regeneration process after cryo-lesion of the peripheral growing zone. Therefore, Prox1 functions in vertebrate retinogenesis may be more complex than previously expected.

Caspase-dependent apoptosis of ON bipolar cells in the goldfish marginal retina

Progenitor cells proliferate at the retinal margin of adult goldfish, so that the developmental stages of maturing retinal cells are arranged in temporal sequence along the periphero-central axis. We studied the caspase-dependent apoptosis of ON bipolar cells, which were labeled by an antibody to protein kinase C (PKC). In both primary and secondary growth zones of the marginal retina, PKC+ cell somata were of two distinct sizes, large and small. A caspase inhibitor (zVAD-fmk), injected into the vitreous, increased the number of both large and small PKC+ somata (times approximately 2 and 1.1, respectively) in both growth zones. These data show that about one-third of immature ON bipolar cells were removed by caspase-dependent apoptosis before being incorporated into the mature retina.