Connexin30.2: In Vitro Interaction with Connexin36 in HeLa Cells and Expression in AII Amacrine Cells and Intrinsically Photosensitive Ganglion Cells in the Mouse Retina

The Retinal Basis of Light Aversion in Neonatal Mice

Aversive responses to bright light (photoaversion) require signaling from the eye to the brain. Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) encode absolute light intensity and are thought to provide the light signals for photoaversion. Consistent with this, neonatal mice exhibit photoaversion before the developmental onset of image vision, and melanopsin deletion abolishes photoaversion in neonates. It is not well understood how the population of ipRGCs, which constitutes multiple physiologically distinct types (denoted M1-M6 in mouse), encodes light stimuli to produce an aversive response. Here, we provide several lines of evidence that M1 ipRGCs that lack the Brn3b transcription factor drive photoaversion in neonatal mice. First, neonatal mice lacking TRPC6 and TRPC7 ion channels failed to turn away from bright light, while two photon Ca2+ imaging of their acutely isolated retinas revealed reduced photosensitivity in M1 ipRGCs, but not other ipRGC types. Second, mice in which all ipRGC types except for Brn3b-negative M1 ipRGCs are ablated exhibited normal photoaversion. Third, pharmacological blockade or genetic knockout of gap junction channels expressed by ipRGCs, which reduces the light sensitivity of M2-M6 ipRGCs in the neonatal retina, had small effects on photoaversion only at the brightest light intensities. Finally, M1s were not strongly depolarized by spontaneous retinal waves, a robust source of activity in the developing retina that depolarizes all other ipRGC types. M1s therefore constitute a separate information channel between the neonatal retina and brain that could ensure behavioral responses to light but not spontaneous retinal waves.SIGNIFICANCE STATEMENT At an early stage of development, before the maturation of photoreceptor input to the retina, neonatal mice exhibit photoaversion. On exposure to bright light, they turn away and emit ultrasonic vocalizations, a cue to their parents to return them to the nest. Neonatal photoaversion is mediated by intrinsically photosensitive retinal ganglion cells (ipRGCs), a small percentage of the retinal ganglion cell population that express the photopigment melanopsin and depolarize directly in response to light. This study shows that photoaversion is mediated by a subset of ipRGCs, called M1-ipRGCs. Moreover, M1-ipRGCs have reduced responses to retinal waves, providing a mechanism by which the mouse distinguishes light stimulation from developmental patterns of spontaneous activity.

Amacrine Cells Forming Gap Junctions With Intrinsically Photosensitive Retinal Ganglion Cells: ipRGC Types, Neuromodulator Contents, and Connexin Isoform

Purpose

Intrinsically photosensitive retinal ganglion cells (ipRGCs) signal not only centrally to non-image-forming visual centers of the brain but also intraretinally to amacrine interneurons through gap junction electrical coupling, potentially modulating image-forming retinal processing. We aimed to determine (1) which ipRGC types couple with amacrine cells, (2) the neuromodulator contents of ipRGC-coupled amacrine cells, and (3) whether connexin36 (Cx36) contributes to ipRGC-amacrine coupling.

Methods

Gap junction-permeable Neurobiotin tracer was injected into green fluorescent protein (GFP)-labeled ipRGCs in Opn4Cre/+; Z/EG mice to stain coupled amacrine cells, and immunohistochemistry was performed to reveal the neuromodulator contents of the Neurobiotin-stained amacrine cells. We also created Opn4Cre/+; Cx36flox/flox; Z/EG mice to knock out Cx36 in GFP-labeled ipRGCs and looked for changes in the number of ipRGC-coupled amacrine cells.

Results

Seventy-three percent of ipRGCs, including all six types (M1-M6), were tracer-coupled with amacrine somas 5.7 to 16.5 µm in diameter but not with ganglion cells. Ninety-two percent of the ipRGC-coupled somas were in the ganglion cell layer and the rest in the inner nuclear layer. Some ipRGC-coupled amacrine cells were found to accumulate serotonin or to contain nitric oxide synthase or neuropeptide Y. Knocking out Cx36 in M2 and M4 dramatically reduced the number of coupled somas.

Conclusions

Heterologous gap junction coupling with amacrine cells is widespread across mouse ipRGC types. ipRGC-coupled amacrine cells probably comprise multiple morphologic types and use multiple neuromodulators, suggesting that gap junctional ipRGC-to-amacrine signaling likely exerts diverse modulatory effects on retinal physiology. ipRGC-amacrine coupling is mediated partly, but not solely, by Cx36.

Phosphorylation of Connexin36 near the C-terminus switches binding affinities for PDZ-domain and 14-3-3 proteins in vitro

Connexin36 (Cx36) is the most abundant connexin in central nervous system neurons. It forms gap junction channels that act as electrical synapses. Similar to chemical synapses, Cx36-containing gap junctions undergo activity-dependent plasticity and complex regulation. Cx36 gap junctions represent multimolecular complexes and contain cytoskeletal, regulatory and scaffolding proteins, which regulate channel conductance, assembly and turnover. The amino acid sequence of mammalian Cx36 harbors a phosphorylation site for the Ca2+/calmodulin-dependent kinase II at serine 315. This regulatory site is homologous to the serine 298 in perch Cx35 and in close vicinity to a PDZ binding domain at the very C-terminal end of the protein. We hypothesized that this phosphorylation site may serve as a molecular switch, influencing the affinity of the PDZ binding domain for its binding partners. Protein microarray and pulldown experiments revealed that this is indeed the case: phosphorylation of serine 298 decreased the binding affinity for MUPP1, a known scaffolding partner of connexin36, and increased the binding affinity for two different 14-3-3 proteins. Although we did not find the same effect in cell culture experiments, our data suggest that phosphorylation of serine 315/298 may serve to recruit different proteins to connexin36/35-containing gap junctions in an activity-dependent manner.

Cx30.2 deletion causes imbalances in testicular Cx43, Cx46, and Cx50 and insulin receptors. Reciprocally, diabetes/obesity alters Cx30.2 in mouse testis

Cx30.2 protein content and localization were assessed during development. An account of Cx30.2, Cx43, Cx46, and Cx50, and insulin receptor (IR) responses to Cx30.2, Cx46, or Cx50 deficiency in mouse interstitial tissue (ITf)- and seminiferous tubule-enriched fractions (STf) is given. The impact of high glucose/insulin on Cx30.2 was investigated in spontaneously diabetic and obese db/db and ob/ob mouse testis and anterior pituitary (AP). Cx30.2 labeled contacts in vascular endothelial and Leydig cells and Sertoli cell junctions in stage V-VII. Cx30.2 expression is regulated differently in the interstitium and tubules. Cx30.2 at 30-kDa levels peaked by 28 days in ITf and by 14 days in STf. In STf, deleting Cx30.2 decreased Cx43 and Cx50, whereas deleting Cx50 downregulated Cx30.2. The opposite occurred in ITf. In STf, deleting Cx30.2 upregulated Cx46 except the full-length reciprocally, deleting Cx46 upregulated Cx30.2. In ITf, Cx30.2 deficiency upregulated full-length and phosphorylated Cx46, whereas deleting Cx46 downregulated 48- to 50-kDa Cx30.2. The db/db and ob/ob mouse ITf, STf, and AP showed imbalanced Cx30.2 levels. IRα levels at 135 kDa declined in Cx30.2-/- and Cx50-/- mouse ITf and Cx46-/- and Cx50-/- STf. IRβ at 98 to 110 kDa dropped in Cx30.2-/- and Cx46-/- mice STf suggesting that Cx30.2 deficiency decreases active IR sites. The results show the connexins interdependence and interaction and that altering a single connexin changes the remaining connexins expression, which can modify gap junction-mediated glucose exchanges in contacting cells. Data suggest that glucose/insulin influences Cx30.2 turnover in testis and AP and, reciprocally, that connexins modulate testis glucose uptake and response to insulin.

Crosstalk: The diversity of melanopsin ganglion cell types has begun to challenge the canonical divide between image-forming and non-image-forming vision

Melanopsin ganglion cells have defied convention since their discovery almost 20 years ago. In the years following, many types of these intrinsically photosensitive retinal ganglion cells (ipRGCs) have emerged. In the mouse retina, there are currently six known types (M1-M6) of melanopsin ganglion cells, each with unique morphology, mosaics, connections, physiology, projections, and functions. While melanopsin-expressing cells are usually associated with behaviors like circadian photoentrainment and the pupillary light reflex, the characterization of multiple types has demonstrated a reach that may extend far beyond non-image-forming vision. In fact, studies have shown that individual types of melanopsin ganglion cells have the potential to impact image-forming functions like contrast sensitivity and color opponency. Thus, the goal of this review is to summarize the morphological and functional aspects of the six known types of melanopsin ganglion cells in the mouse retina and to highlight their respective roles in non-image-forming and image-forming vision. Although many melanopsin ganglion cell types do project to image-forming brain targets, it is important to note that this is only the first step in determining their influence on image-forming vision. Even so, the visual system has canonically been divided into these two functional realms and melanopsin ganglion cells have begun to challenge the boundary between them, providing an overlap of visual information that is complementary rather than redundant. Further studies on these ganglion cell photoreceptors will no doubt continue to illustrate an ever-expanding role for melanopsin ganglion cells in image-forming vision.

Localization of Retinal Ca2+/Calmodulin-Dependent Kinase II-β (CaMKII-β) at Bipolar Cell Gap Junctions and Cross-Reactivity of a Monoclonal Anti-CaMKII-β Antibody With Connexin36

Neuronal gap junctions formed by connexin36 (Cx36) and chemical synapses share striking similarities in terms of plasticity. Ca²⁺/calmodulin-dependent protein kinase II (CaMKII), an enzyme known to induce memory formation at chemical synapses, has recently been described to potentiate electrical coupling in the retina and several other brain areas via phosphorylation of Cx36. The contribution of individual CaMKII isoforms to this process, however, remains unknown. We recently identified CaMKII-β at electrical synapses in the mouse retina. Now, we set out to identify cell types containing Cx36 gap junctions that also express CaMKII-β. To ensure precise description, we first tested the specificity of two commercially available antibodies on CaMKII-β-deficient retinas. We found that a polyclonal antibody was highly specific for CaMKII-β. However, a monoclonal antibody (CB-β-1) recognized CaMKII-β but also cross-reacted with the C-terminal tail of Cx36, making localization analyses with this antibody inaccurate. Using the polyclonal antibody, we identified strong CaMKII-β expression in bipolar cell terminals that were secretagogin- and HCN1-positive and thus represent terminals of type 5 bipolar cells. In these terminals, a small fraction of CaMKII-β also colocalized with Cx36. A similar pattern was observed in putative type 6 bipolar cells although there, CaMKII expression seemed less pronounced. Next, we tested whether CaMKII-β influenced the Cx36 expression in bipolar cell terminals by quantifying the number and size of Cx36-immunoreactive puncta in CaMKII-β-deficient retinas. However, we found no significant differences between the genotypes, indicating that CaMKII-β is not necessary for the formation and maintenance of Cx36-containing gap junctions in the retina. In addition, in wild-type retinas, we observed frequent association of Cx36 and CaMKII-β with synaptic ribbons, i.e., chemical synapses, in bipolar cell terminals. This arrangement resembled the composition of mixed synapses found for example in Mauthner cells, in which electrical coupling is regulated by glutamatergic activity. Taken together, our data imply that CaMKII-β may fulfill several functions in bipolar cell terminals, regulating both Cx36-containing gap junctions and ribbon synapses and potentially also mediating cross-talk between these two types of bipolar cell outputs.

Gap Junctions in A8 Amacrine Cells Are Made of Connexin36 but Are Differently Regulated Than Gap Junctions in AII Amacrine Cells

In the mammalian retina, amacrine cells represent the most diverse cell class and are involved in the spatio-temporal processing of visual signals in the inner plexiform layer. They are connected to bipolar, other amacrine and ganglion cells, forming complex networks via electrical and chemical synapses. The small-field A8 amacrine cell was shown to receive non-selective glutamatergic input from OFF and ON cone bipolar cells at its bistratified dendrites in sublamina 1 and 4 of the inner plexiform layer. Interestingly, it was also shown to form electrical synapses with ON cone bipolar cells, thus resembling the rod pathway-specific AII amacrine cell. In contrast to the AII cell, however, the electrical synapses of A8 cells are poorly understood. Therefore, we made use of the Ier5-GFP mouse line, in which A8 cells are labeled by GFP, to study the gap junction composition and frequency in A8 cells. We found that A8 cells form <20 gap junctions per cell and these gap junctions consist of connexin36. Connexin36 is present at both OFF and ON dendrites of A8 cells, preferentially connecting A8 cells to type 1 OFF and type 6 and 7 ON bipolar cells and presumably other amacrine cells. Additionally, we show that the OFF dendrites of A8 cells co-stratify with the processes of dopaminergic amacrine cells from which they may receive GABAergic input via GABA_A receptor subunit α3. As we found A8 cells to express dopamine receptor D₁ (but not D₂), we also tested whether A8 cell coupling is modulated by D₁ receptor agonists and antagonists as was shown for the coupling of AII cells. However, this was not the case. In summary, our data suggests that A8 coupling is differently regulated than AII cells and may even be independent of ambient light levels and serve signal facilitation rather than providing a separate neuronal pathway.

Extrasynaptic NMDA Receptors on Rod Pathway Amacrine Cells: Molecular Composition, Activation, and Signaling

In the rod pathway of the mammalian retina, axon terminals of glutamatergic rod bipolar cells are presynaptic to AII and A17 amacrine cells in the inner plexiform layer. Recent evidence suggests that both amacrines express NMDA receptors, raising questions concerning molecular composition, localization, activation, and function of these receptors. Using dual patch-clamp recording from synaptically connected rod bipolar and AII or A17 amacrine cells in retinal slices from female rats, we found no evidence that NMDA receptors contribute to postsynaptic currents evoked in either amacrine. Instead, NMDA receptors on both amacrine cells were activated by ambient glutamate, and blocking glutamate uptake increased their level of activation. NMDA receptor activation also increased the frequency of GABAergic postsynaptic currents in rod bipolar cells, suggesting that NMDA receptors can drive release of GABA from A17 amacrines. A striking dichotomy was revealed by pharmacological and immunolabeling experiments, which found GluN2B-containing NMDA receptors on AII amacrines and GluN2A-containing NMDA receptors on A17 amacrines. Immunolabeling also revealed a clustered organization of NMDA receptors on both amacrines and a close spatial association between GluN2B subunits and connexin 36 on AII amacrines, suggesting that NMDA receptor modulation of gap junction coupling between these cells involves the GluN2B subunit. Using multiphoton Ca2+ imaging, we verified that activation of NMDA receptors evoked an increase of intracellular Ca2+ in dendrites of both amacrines. Our results suggest that AII and A17 amacrines express clustered, extrasynaptic NMDA receptors, with different and complementary subunits that are likely to contribute differentially to signal processing and plasticity.SIGNIFICANCE STATEMENT Glutamate is the most important excitatory neurotransmitter in the CNS, but not all glutamate receptors transmit fast excitatory signals at synapses. NMDA-type glutamate receptors act as voltage- and ligand-gated ion channels, with functional properties determined by their specific subunit composition. These receptors can be found at both synaptic and extrasynaptic sites on neurons, but the role of extrasynaptic NMDA receptors is unclear. Here, we demonstrate that retinal AII and A17 amacrine cells, postsynaptic partners at rod bipolar dyad synapses, express extrasynaptic (but not synaptic) NMDA receptors, with different and complementary GluN2 subunits. The localization of GluN2A-containing receptors to A17s and GluN2B-containing receptors to AIIs suggests a mechanism for differential modulation of excitability and signaling in this retinal microcircuit.

Electrotonic signal processing in AII amacrine cells: compartmental models and passive membrane properties for a gap junction-coupled retinal neuron

Amacrine cells are critical for processing of visual signals, but little is known about their electrotonic structure and passive membrane properties. AII amacrine cells are multifunctional interneurons in the mammalian retina and essential for both rod- and cone-mediated vision. Their dendrites are the site of both input and output chemical synapses and gap junctions that form electrically coupled networks. This electrical coupling is a challenge for developing realistic computer models of single neurons. Here, we combined multiphoton microscopy and electrophysiological recording from dye-filled AII amacrine cells in rat retinal slices to develop morphologically accurate compartmental models. Passive cable properties were estimated by directly fitting the current responses of the models evoked by voltage pulses to the physiologically recorded responses, obtained after blocking electrical coupling. The average best-fit parameters (obtained at - 60 mV and ~ 25 °C) were 0.91 µF cm^-2 for specific membrane capacitance, 198 Ω cm for cytoplasmic resistivity, and 30 kΩ cm² for specific membrane resistance. We examined the passive signal transmission between the cell body and the dendrites by the electrotonic transform and quantified the frequency-dependent voltage attenuation in response to sinusoidal current stimuli. There was significant frequency-dependent attenuation, most pronounced for signals generated at the arboreal dendrites and propagating towards the soma and lobular dendrites. In addition, we explored the consequences of the electrotonic structure for interpreting currents in somatic, whole-cell voltage-clamp recordings. The results indicate that AII amacrines cannot be characterized as electrotonically compact and suggest that their morphology and passive properties can contribute significantly to signal integration and processing.

Differential Distribution of Retinal Ca2+/Calmodulin-Dependent Kinase II (CaMKII) Isoforms Indicates CaMKII-β and -δ as Specific Elements of Electrical Synapses Made of Connexin36 (Cx36)

AII amacrine cells are essential interneurons of the primary rod pathway and transmit rod-driven signals to ON cone bipolar cells to enable scotopic vision. Gap junctions made of connexin36 (Cx36) mediate electrical coupling among AII cells and between AII cells and ON cone bipolar cells. These gap junctions underlie a remarkable degree of plasticity and are modulated by different signaling cascades. In particular, Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) has been characterized as an important regulator of Cx36, capable of potentiating electrical coupling in AII cells. However, it is unclear which CaMKII isoform mediates this effect. To obtain a more detailed understanding of the isoform composition of CaMKII at retinal gap junctions, we analyzed the retinal distribution of all four CaMKII isoforms using confocal microscopy. These experiments revealed a differential distribution of CaMKII isoforms: CaMKII-α was strongly expressed in starburst amacrine cells, which are known to lack electrical coupling. CaMKII-β was abundant in OFF bipolar cells, which form electrical synapses in the outer and the inner retina. CaMKII-γ was diffusely distributed across the entire retina and could not be assigned to a specific cell type. CaMKII-δ labeling was evident in bipolar and AII amacrine cells, which contain the majority of Cx36-immunoreactive puncta in the inner retina. We double-labeled retinas for Cx36 and the four CaMKII isoforms and revealed that the composition of the CaMKII enzyme differs between gap junctions in the outer and the inner retina: in the outer retina, only CaMKII-β colocalized with Cx36-containing gap junctions, whereas in the inner retina, CaMKII-β and -δ colocalized with Cx36. This finding suggests that gap junctions in the inner and the outer retina may be regulated differently although they both contain the same connexin. Taken together, our study identifies CaMKII-β and -δ as Cx36-specific regulators in the mouse retina with CaMKII-δ regulating the primary rod pathway.

Classification of Mouse Retinal Bipolar Cells: Type-Specific Connectivity with Special Reference to Rod-Driven AII Amacrine Pathways

We confirmed the classification of 15 morphological types of mouse bipolar cells by serial section transmission electron microscopy and characterized each type by identifying chemical synapses and gap junctions at axon terminals. Although whether the previous type 5 cells consist of two or three types was uncertain, they are here clustered into three types based on the vertical distribution of axonal ribbons. Next, while two groups of rod bipolar (RB) cells, RB1, and RB2, were previously proposed, we clarify that a half of RB1 cells have the intermediate characteristics, suggesting that these two groups comprise a single RB type. After validation of bipolar cell types, we examined their relationship with amacrine cells then particularly with AII amacrine cells. We found a strong correlation between the number of amacrine cell synaptic contacts and the number of bipolar cell axonal ribbons. Formation of bipolar cell output at each ribbon synapse may be effectively regulated by a few nearby inhibitory inputs of amacrine cells which are chosen from among many amacrine cell types. We also found that almost all types of ON cone bipolar cells frequently have a minor group of midway ribbons along the axon passing through the OFF sublamina as well as a major group of terminal ribbons in the ON sublamina. AII amacrine cells are connected to five of six OFF bipolar cell types via conventional chemical synapses and seven of eight ON (cone) bipolar cell types via electrical synapses (gap junctions). However, the number of synapses is dependent on bipolar cell types. Type 2 cells have 69% of the total number of OFF bipolar chemical synaptic contacts with AII amacrine cells and type 6 cells have 46% of the total area of ON bipolar gap junctions with AII amacrine cells. Both type 2 and 6 cells gain the greatest access to AII amacrine cell signals also share those signals with other types of bipolar cells via networked gap junctions. These findings imply that the most sensitive scotopic signal may be conveyed to the center by ganglion cells that have the most numerous synapses with type 2 and 6 cells.

Prox1 Is a Marker for AII Amacrine Cells in the Mouse Retina

The transcription factor Prox1 is expressed in multiple cells in the retina during eye development. This study has focused on neuronal Prox1 expression in the inner nuclear layer (INL) of the adult mouse retina. Prox1 immunostaining was evaluated in vertical retinal sections and whole mount preparations using a specific antibody directed to the C-terminus of Prox1. Strong immunostaining was observed in numerous amacrine cell bodies and in all horizontal cell bodies in the proximal and distal INL, respectively. Some bipolar cells were also weakly immunostained. Prox1-immunoreactive amacrine cells expressed glycine, and they formed 35 ± 3% of all glycinergic amacrine cells. Intracellular Neurobiotin injections into AII amacrine cells showed that all gap junction-coupled AII amacrine cells express Prox1, and no other Prox1-immunostained amacrine cells were in the immediate area surrounding the injected AII amacrine cell. Prox1-immunoreactive amacrine cell bodies were distributed across the retina, with their highest density (3887 ± 160 cells/mm²) in the central retina, 0.5 mm from the optic nerve head, and their lowest density (3133 ± 350 cells/mm²) in the mid-peripheral retina, 2 mm from the optic nerve head. Prox1-immunoreactive amacrine cell bodies comprised ~9.8% of the total amacrine cell population, and they formed a non-random mosaic with a regularity index (RI) of 3.4, similar to AII amacrine cells in the retinas of other mammals. Together, these findings indicate that AII amacrine cells are the predominant and likely only amacrine cell type strongly expressing Prox1 in the adult mouse retina, and establish Prox1 as a marker of AII amacrine cells.