Spinule formulation in the fish retina: is there an involvement of actin and tubulin? An electronmicroscopic immunogold study

P-Rex2, a Rac-guanine nucleotide exchange factor, is expressed selectively in ribbon synaptic terminals of the mouse retina

Background

Phosphatidylinositol (3,4,5)-trisphosphate-dependent Rac Exchanger 2 (P-Rex2) is a guanine nucleotide exchange factor (GEF) that specifically activates Rac GTPases, important regulators of actin cytoskeleton remodeling. P-Rex2 is known to modulate cerebellar Purkinje cell architecture and function, but P-Rex2 expression and function elsewhere in the central nervous system is unclear. To better understand potential roles for P-Rex2 in neuronal cytoskeletal remodeling and function, we performed widefield and confocal microscopy of specimens double immunolabeled for P-Rex2 and cell- and synapse-specific markers in the mouse retina.

Results

P-Rex2 was restricted to the plexiform layers of the retina and colocalized extensively with Vesicular Glutamate Transporter 1 (VGluT1), a specific marker for photoreceptor and bipolar cell terminals. Double labeling for P-Rex2 and peanut agglutinin, a cone terminal marker, confirmed that P-Rex2 was present in both rod and cone terminals. Double labeling with markers for specific bipolar cell types showed that P-Rex2 was present in the terminals of rod bipolar cells and multiple ON- and OFF-cone bipolar cell types. In contrast, P-Rex2 was not expressed in the processes or conventional synapses of amacrine or horizontal cells.

Conclusions

P-Rex2 is associated specifically with the glutamatergic ribbon synaptic terminals of photoreceptors and bipolar cells that transmit visual signals vertically through the retina. The Rac-GEF function of P-Rex2 implies a specific role for P-Rex2 and Rac-GTPases in regulating the actin cytoskeleton in glutamatergic ribbon synaptic terminals of retinal photoreceptors and bipolar cells and appears to be ideally positioned to modulate the adaptive plasticity of these terminals.

Calcium-binding protein Caldendrin and CaMKII are localized in spinules of the carp retina

Calcium-binding proteins translate the influx of Ca(2+) at excitatory synapses into spatiotemporal signals that regulate a variety of processes underlying synaptic plasticity. In the fish retina, the synaptic connectivity between photoreceptors and horizontal cells undergoes a remarkable plasticity, triggered by the ambient light conditions. With increasing light, the synaptic dendrites of horizontal cells form numerous spinules that are dissolved during dark adaptation. The dynamic regulation of this process is calcium-dependent and involves the Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), but astonishingly its principal regulator Calmodulin (CaM) could not be localized to spinules. Here, we show that antibodies directed against Caldendrin (CaBP1), a member of the EF-hand calcium-binding protein family, strongly label the terminal dendrites of horizontal cells invaginating cone pedicles. Double-labeling experiments revealed that this label is closely associated with label for CaMKII. This association was confirmed at the ultrastructural level. Caldendrin immunoreactivity and CaMKII immunoreactivity are both present in horizontal cell dendrites flanking the synaptic ribbon within the cone pedicle and in particular in spinules formed by these terminals. Comparison of light- and dark-adapted retinas revealed a shift of the membrane-associated label for Caldendrin from the terminal dendrites into the spinules during light adaptation. These results suggest that Caldendrin is involved in the dynamic regulation of spinules and confirms the assumed potential of Caldendrin as a neural calcium sensor for synaptic plasticity.

Calcium and protein kinase C regulate the actin cytoskeleton in the synaptic terminal of retinal bipolar cells

The organization of filamentous actin (F-actin) in the synaptic pedicle of depolarizing bipolar cells from the goldfish retina was studied using fluorescently labeled phalloidin. The amount of F-actin in the synaptic pedicle relative to the cell body increased from a ratio of 1.6 +/- 0.1 in the dark to 2.1 +/- 0.1 after exposure to light. Light also caused the retraction of spinules and processes elaborated by the synaptic pedicle in the dark. Isolated bipolar cells were used to characterize the factors affecting the actin cytoskeleton. When the electrical effect of light was mimicked by depolarization in 50 mM K+, the actin network in the synaptic pedicle extended up to 2.5 micrometer from the plasma membrane. Formation of F-actin occurred on the time scale of minutes and required Ca2+ influx through L-type Ca2+ channels. Phorbol esters that activate protein kinase C (PKC) accelerated growth of F-actin. Agents that inhibit PKC hindered F-actin growth in response to Ca2+ influx and accelerated F-actin breakdown on removal of Ca2+. To test whether activity-dependent changes in the organization of F-actin might regulate exocytosis or endocytosis, vesicles were labeled with the fluorescent membrane marker FM1-43. Disruption of F-actin with cytochalasin D did not affect the continuous cycle of exocytosis and endocytosis that was stimulated by maintained depolarization, nor the spatial distribution of recycled vesicles within the synaptic terminal. We suggest that the actions of Ca2+ and PKC on the organization of F-actin regulate the morphology of the synaptic pedicle under varying light conditions.

Interocular effect of actin depolymerization on spinule formation in teleost retina

Teleost retinas adapted to light show numerous spinules invaginated in the cone pedicles whereas darkness induces a reduction in the number of spinules. Horizontal cells show nematosomes whose size decreases as the number of spinules increases. We have investigated the involvement of actin filaments in spinule formation, by using cytochalasin D through intraocular injection into an eye. The ultrastructural analysis reveals that cytochalasin D impairs spinule formation and nematosome-size reduction in both treated and contralateral untreated retinas.

Evidence for calcium/calmodulin dependence of spinule retraction in retinal horizontal cells

Horizontal cells of the carp retina alter their synaptic connections with cones during dark and light adaptation. At light onset, dendrites of horizontal cells, which are positioned laterally at the ribbon synapse, form "spinules," little processes with membrane densities. Spinules are retracted again during dark adaptation. Spinule retraction is also elicited upon glutamate application to the retina. In the present study, we address the question whether calcium/calmodulin-dependent pathways are involved in dark- and glutamate-evoked spinule retraction. Light-adapted retinas were isolated and subsequently dark adapted during incubation in media of different calcium concentrations. Spinule retraction was clearly blocked in low-calcium solutions (5 microM and 50 nM CaCl2). Incubation in medium containing cobalt chloride (2 mM) had the same effect. Both treatments blocked the glutamate-induced spinule retraction as well. These results indicate that spinule retraction is induced by a calcium influx into horizontal cells. To investigate whether calmodulin, the primary calcium receptor in eukaryotic cells, is present at the site of spinule formation, light- and dark-adapted retinas, embedded in LR White resin, were labelled with an antibody against calmodulin and gold-conjugated secondary antibodies. Horizontal cell dendrites at the ribbon synapse revealed strong calmodulin immunoreactivity, which was more than twice as high in light- as in dark-adapted retinas. The incubation of isolated retinas with the calmodulin antagonists W5 and W13 inhibited spinule retraction. In summary, these results suggest that spinule retraction may be regulated by calcium influx into horizontal cells and subsequent calcium/calmodulin-dependent pathways.