Signal transmission at invaginating cone photoreceptor synaptic contacts following deletion of the presynaptic cytomatrix protein Bassoon in mouse retina

GRF2 Is Crucial for Cone Photoreceptor Viability and Ribbon Synapse Formation in the Mouse Retina

Using constitutive GRF1/2 knockout mice, we showed previously that GRF2 is a key regulator of nuclear migration in retinal cone photoreceptors. To evaluate the functional relevance of that cellular process for two putative targets of the GEF activity of GRF2 (RAC1 and CDC42), here we compared the structural and functional retinal phenotypes resulting from conditional targeting of RAC1 or CDC42 in the cone photoreceptors of constitutive GRF2KO and GRF2WT mice. We observed that single RAC1 disruption did not cause any obvious morphological or physiological changes in the retinas of GRF2WT mice, and did not modify either the phenotypic alterations previously described in the retinal photoreceptor layer of GRF2KO mice. In contrast, the single ablation of CDC42 in the cone photoreceptors of GRF2WT mice resulted in clear alterations of nuclear movement that, unlike those of the GRF2KO retinas, were not accompanied by electrophysiological defects or slow, progressive cone cell degeneration. On the other hand, the concomitant disruption of GRF2 and CDC42 in the cone photoreceptors resulted, somewhat surprisingly, in a normalized pattern of nuclear positioning/movement, similar to that physiologically observed in GRF2WT mice, along with worsened patterns of electrophysiological responses and faster rates of cell death/disappearance than those previously recorded in single GRF2KO cone cells. Interestingly, the increased rates of cone cell apoptosis/death observed in single GRF2KO and double-knockout GRF2KO/CDC42KO retinas correlated with the electron microscopic detection of significant ultrastructural alterations (flattening) of their retinal ribbon synapses that were not otherwise observed at all in single-knockout CDC42KO retinas. Our observations identify GRF2 and CDC42 (but not RAC1) as key regulators of retinal processes controlling cone photoreceptor nuclear positioning and survival, and support the notion of GRF2 loss-of-function mutations as potential drivers of cone retinal dystrophies.

Impact of the Voltage-Gated Calcium Channel Antagonist Nimodipine on the Development of Oligodendrocyte Precursor Cells

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system (CNS). While most of the current treatment strategies focus on immune cell regulation, except for the drug siponimod, there is no therapeutic intervention that primarily aims at neuroprotection and remyelination. Recently, nimodipine showed a beneficial and remyelinating effect in experimental autoimmune encephalomyelitis (EAE), a mouse model of MS. Nimodipine also positively affected astrocytes, neurons, and mature oligodendrocytes. Here we investigated the effects of nimodipine, an L-type voltage-gated calcium channel antagonist, on the expression profile of myelin genes and proteins in the oligodendrocyte precursor cell (OPC) line Oli-Neu and in primary OPCs. Our data indicate that nimodipine does not have any effect on myelin-related gene and protein expression. Furthermore, nimodipine treatment did not result in any morphological changes in these cells. However, RNA sequencing and bioinformatic analyses identified potential micro (mi)RNA that could support myelination after nimodipine treatment compared to a dimethyl sulfoxide (DMSO) control. Additionally, we treated zebrafish with nimodipine and observed a significant increase in the number of mature oligodendrocytes (* p≤ 0.05). Taken together, nimodipine seems to have different positive effects on OPCs and mature oligodendrocytes.

Synaptic vesicle release during ribbon synapse formation of cone photoreceptors

Mammalian cone photoreceptors enable through their sophisticated synapse the high-fidelity transfer of visual information to second-order neurons in the retina. The synapse contains a proteinaceous organelle, called the synaptic ribbon, which tethers synaptic vesicles (SVs) at the active zone (AZ) close to voltage-gated Ca2+ channels. However, the exact contribution of the synaptic ribbon to neurotransmission is not fully understood, yet. In mice, precursors to synaptic ribbons appear within photoreceptor terminals shortly after birth as free-floating spherical structures, which progressively elongate and then attach to the AZ during the following days. Here, we took advantage of the process of synaptic ribbon maturation to study their contribution to SV release. We performed whole-cell patch-clamp recordings from cone photoreceptors at three postnatal (P) development stages (P8-9, P12-13, >P30) and measured evoked SV release, SV replenishment rate, recovery from synaptic depression, domain organization of voltage-sensitive Ca2+ channels, and Ca2+-sensitivity of exocytosis. Additionally, we performed electron microscopy to determine the density of SVs at ribbon-free and ribbon-occupied AZs. Our results suggest that ribbon attachment does not organize the voltage-sensitive Ca2+ channels into nanodomains or control SV release probability. However, ribbon attachment increases SV density at the AZ, increases the pool size of readily releasable SVs available for evoked SV release, facilitates SV replenishment without changing the SV pool refilling time, and increases the Ca2+- sensitivity of glutamate release.

Bassoon controls synaptic vesicle release via regulation of presynaptic phosphorylation and cAMP

Neuronal presynaptic terminals contain hundreds of neurotransmitter‐filled synaptic vesicles (SVs). The morphologically uniform SVs differ in their release competence segregating into functional pools that differentially contribute to neurotransmission. The presynaptic scaffold bassoon is required for neurotransmission, but the underlying molecular mechanisms are unknown. We report that glutamatergic synapses lacking bassoon feature decreased SV release competence and increased resting pool of SVs as assessed by imaging of SV release in cultured neurons. CDK5/calcineurin and cAMP/PKA presynaptic signalling are dysregulated, resulting in an aberrant phosphorylation of their downstream effectors synapsin1 and SNAP25, well‐known regulators of SV release competence. An acute pharmacological restoration of physiological CDK5 and cAMP/PKA activity fully normalises the SV pools in neurons lacking bassoon. Finally, we demonstrate that CDK5‐dependent regulation of PDE4 activity interacts with cAMP/PKA signalling and thereby controls SV release competence. These data reveal that bassoon organises SV pools in glutamatergic synapses via regulation of presynaptic phosphorylation and cAMP homeostasis and indicate a role of CDK5/PDE4/cAMP axis in the control of neurotransmitter release.

Intravitreal and subretinal syngeneic bone marrow mononuclear stem cell transplantation improves photoreceptor survival but does not ameliorate retinal function in two rat models of retinal degeneration

PURPOSE

To study and compare effects of syngeneic bone marrow mononuclear stem cells (BM-MNCs) transplants on inherited retinal degeneration in two animal models with different etiologies: the RCS and the P23H-1 rats. To compare the safety and efficacy of two methods of intraocular delivery: subretinal and/or intravitreal.

METHODS

A suspension of BM-MNCs was injected subretinally or intravitreally in the left eyes of P23H-1 and RCS rats at post-natal day (P) 21. At different survival intervals after the injection: 7, 15, 30 or 60 days, the retinas were cross-sectioned, and photoreceptor survival and glial cell responses were investigated using immunodetection of cones (anti-cone arrestin), synaptic connections (anti-bassoon), microglia (anti-Iba-1), astrocytes and Müller cells (anti-GFAP). Electroretinographic function was also assessed longitudinally.

RESULTS

Intravitreal injections (IVIs) or subretinal injections (SRIs) of BM-MNCs did not produce adverse effects. The transplanted cells survived for up to 15 days but did not penetrate the retina. Both IVIs and SRIs increased photoreceptor survival, decreased synaptic degeneration and glial fibrillary acidic protein (GFAP) expression in Müller cells but did not modify microglial cell activation and migration or the electroretinographic responses.

CONCLUSIONS

Intravitreal and subretinal syngeneic BM-MNCs transplantation decreases photoreceptor degeneration and shows anti-gliotic effects on Müller cells but does not ameliorate retinal function. Moreover, syngeneic BM-MNCs transplants are more effective than the xenotransplants of these cells. BM-MNC transplantation has potential therapeutic effects that merit further investigation.

Wnt inhibitory 1 ameliorates neovascularization and attenuates photoreceptor injury in an oxygen-induced retinopathy mouse model

Retinal neovascularization (RNV) associated diseases typically exhibit pathological neovascularization and neurodegeneration. Wnt inhibitor factor 1 (WIF1) is a secreted Wnt antagonist that regulates angiogenesis. However, the significance of WIF1 in RNV associated disease has not been explicitly tested. In our study, we found that the WIF1 expressions were strongly downregulated in the vitreous of proliferative diabetic retinopathy (PDR) and retinopathy of prematurity (ROP). Similarly, retinal WIF1 expression was significantly downregulated in OIR mice, relative to normal mice at P17. After injection of WIF1 overexpression lentivirus into the vitreous of OIR mice, overexpressing WIF1 in OIR mice vitreous strongly reduced avascular areas and neovascular tufts, increased vessel branches, raised a-, b-waves and oscillatory potentials amplitudes on ERG, increased retinal thickness and the number of synapses in retina, normalized the Golgi, mitochondria, and outer segments of photoreceptors. Furthermore, overexpression WIF1 suppressed expressions of β-catenin, vascular endothelial growth factor (VEGF), p-AKT and p-ERK, reduced retinal reactive oxygen species (ROS) and 4-HNE levels, improved autophagic flux, and mitigated apoptosis. In summary, WIF1 plays a key role in alleviating angiogenesis and in improving visual function in OIR mice by suppressing the Wnt/β-catenin-VEGF signaling pathway and ROS levels. WIF1 is an excellent candidate for targeted therapy against RNV associated diseases.

Nimodipine Exerts Beneficial Effects on the Rat Oligodendrocyte Cell Line OLN-93

Multiple sclerosis (MS) is a chronic autoimmune disease of the central nervous system (CNS). Therapy is currently limited to drugs that interfere with the immune system; treatment options that primarily mediate neuroprotection and prevent neurodegeneration are not available. Here, we studied the effects of nimodipine on the rat cell line OLN-93, which resembles young mature oligodendrocytes. Nimodipine is a dihydropyridine that blocks the voltage-gated L-type calcium channel family members Cav1.2 and Cav1.3. Our data show that the treatment of OLN-93 cells with nimodipine induced the upregulation of myelin genes, in particular of proteolipid protein 1 (Plp1), which was confirmed by a significantly greater expression of PLP1 in immunofluorescence analysis and the presence of myelin structures in the cytoplasm at the ultrastructural level. Whole-genome RNA sequencing additionally revealed the upregulation of genes that are involved in neuroprotection, remyelination, and antioxidation pathways. Interestingly, the observed effects were independent of Cav1.2 and Cav1.3 because OLN-93 cells do not express these channels, and there was no measurable response pattern in patch-clamp analysis. Taking into consideration previous studies that demonstrated a beneficial effect of nimodipine on microglia, our data support the notion that nimodipine is an interesting drug candidate for the treatment of MS and other demyelinating diseases.

Functional and Structural Development of Mouse Cone Photoreceptor Ribbon Synapses

Purpose

Cone photoreceptors of the retina use a sophisticated ribbon-containing synapse to convert light-dependent changes in membrane potential into release of synaptic vesicles (SVs). We aimed to study the functional and structural maturation of mouse cone photoreceptor ribbon synapses during postnatal development and to investigate the role of the synaptic ribbon in SV release.

Methods

We performed patch-clamp recordings from cone photoreceptors and their postsynaptic partners, the horizontal cells during postnatal retinal development to reveal the functional parameters of the synapses. To investigate the occurring structural changes, we applied immunocytochemistry and electron microscopy.

Results

We found that immature cone photoreceptor terminals were smaller, they had fewer active zones (AZs) and AZ-anchored synaptic ribbons, and they produced a smaller Ca2+ current than mature photoreceptors. The number of postsynaptic horizontal cell contacts to synaptic terminals increased with age. However, tonic and spontaneous SV release at synaptic terminals stayed similar during postnatal development. Multiquantal SV release was present in all age groups, but mature synapses produced larger multiquantal events than immature ones. Remarkably, at single AZs, tonic SV release was attenuated during maturation and showed an inverse relationship with the appearance of anchored synaptic ribbons.

Conclusions

Our developmental study suggests that the presence of synaptic ribbons at the AZs attenuates tonic SV release and amplifies multiquantal SV release. However, spontaneous SV release may not depend on the presence of synaptic ribbons or voltage-sensitive Ca2+ channels at the AZs.

Genetic disruption of bassoon in two mutant mouse lines causes divergent retinal phenotypes

Bassoon (BSN) is a presynaptic cytomatrix protein ubiquitously present at chemical synapses of the central nervous system, where it regulates synaptic vesicle replenishment and organizes voltage-gated Ca²⁺ channels. In sensory photoreceptor synapses, BSN additionally plays a decisive role in anchoring the synaptic ribbon, a presynaptic organelle and functional extension of the active zone, to the presynaptic membrane. In this study, we functionally and structurally analyzed two mutant mouse lines with a genetic disruption of Bsn-Bsn^gt and Bsn^ko -using electrophysiology and high-resolution microscopy. In both Bsn mutant mouse lines, full-length BSN was abolished, and photoreceptor synaptic function was similarly impaired, yet synapse structure was more severely affected in Bsn^gt/gt than in Bsn^ko/ko photoreceptors. The synaptic defects in Bsn^gt/gt retina coincide with remodeling of the outer retina-rod bipolar and horizontal cell sprouting, formation of ectopic ribbon synaptic sites-and death of cone photoreceptors, processes that did not occur in Bsn^ko/ko retina. An analysis of Bsn^gt/ko hybrid mice revealed that the divergent retinal phenotypes of Bsn^gt/gt and Bsn^ko/ko mice can be attributed to the expression of the Bsn^gt allele, which triggers cone photoreceptor death and neurite sprouting in the outer retina. These findings shed new light on the existing Bsn mutant mouse models and might help to understand mechanisms that drive photoreceptor death.

Transmission at rod and cone ribbon synapses in the retina

Light-evoked voltage responses of rod and cone photoreceptor cells in the vertebrate retina must be converted to a train of synaptic vesicle release events for transmission to downstream neurons. This review discusses the processes, proteins, and structures that shape this critical early step in vision, focusing on studies from salamander retina with comparisons to other experimental animals. Many mechanisms are conserved across species. In cones, glutamate release is confined to ribbon release sites although rods are also capable of release at non-ribbon sites. The role of non-ribbon release in rods remains unclear. Release from synaptic ribbons in rods and cones involves at least three vesicle pools: a readily releasable pool (RRP) matching the number of membrane-associated vesicles along the ribbon base, a ribbon reserve pool matching the number of additional vesicles on the ribbon, and an enormous cytoplasmic reserve. Vesicle release increases in parallel with Ca²⁺ channel activity. While the opening of only a few Ca²⁺ channels beneath each ribbon can trigger fusion of a single vesicle, sustained release rates in darkness are governed by the rate at which the RRP can be replenished. The number of vacant release sites, their functional status, and the rate of vesicle delivery in turn govern replenishment. Along with an overview of the mechanisms of exocytosis and endocytosis, we consider specific properties of ribbon-associated proteins and pose a number of remaining questions about this first synapse in the visual system.

The absence of functional bassoon at cone photoreceptor ribbon synapses affects signal transmission at Off cone bipolar cell contacts in mouse retina

AIM

Off cone bipolar cells of the mammalian retina connect to cone photoreceptor synaptic terminals via non-invaginating flat contacts at a considerable distance from the only established neurotransmitter release site so far, the synaptic ribbon. Diffusion from the ribbon synaptic active zone is considered the most likely mechanism for the neurotransmitter glutamate to reach postsynaptic receptors on the dendritic tips of Off cone bipolar cells. We used a mutant mouse with functionally impaired photoreceptor ribbon synapses to investigate the importance of intact ribbon synaptic active zones for signal transmission at Off cone bipolar cell contacts.

METHODS

Whole-cell patch-clamp recordings from Off cone bipolar cells in a horizontal slice preparation of wildtype (Bsn^wt ) and mutant (Bsn^ΔEx4/5 ) mouse retina were applied to investigate signal transmission between cone photoreceptors and Off cone bipolar cells. The distribution of postsynaptic glutamate receptors in Off cone bipolar cell dendrites was studied using multiplex immunocytochemistry.

RESULTS

Tonic synaptic activity and evoked release were significantly reduced in mutant animals. Vesicle replenishment rates and the size of the readily releasable pool were likewise decreased. The precisely timed transient current response to light offset changed to a sustained response in the mutant, exemplified by random release events only loosely time-locked to the stimulus. The kainate receptor distribution in postsynaptic Off cone bipolar cell dendritic contacts in Bsn^ΔEx4/5 mice was largely disturbed.

CONCLUSION

Our results suggest a major role of functional ribbon synaptic active zones for signal transmission and postsynaptic glutamate receptor organization at flat Off cone bipolar cell contacts.

Function of cone and cone-related pathways in CaV1.4 IT mice

Ca_V1.4 L-type calcium channels are predominantly expressed in photoreceptor terminals playing a crucial role for synaptic transmission and, consequently, for vision. Human mutations in the encoding gene are associated with congenital stationary night blindness type-2. Besides rod-driven scotopic vision also cone-driven photopic responses are severely affected in patients. The present study therefore examined functional and morphological changes in cones and cone-related pathways in mice carrying the Ca_V1.4 gain-of function mutation I756T (Ca_V1.4-IT) using multielectrode array, patch-clamp and immunohistochemical analyses. Ca_V1.4-IT ganglion cell responses to photopic stimuli were seen only in a small fraction of cells indicative of a major impairment in the cone pathway. Though cone photoreceptors underwent morphological rearrangements, they retained their ability to release glutamate. Our functional data suggested a postsynaptic cone bipolar cell defect, supported by the fact that the majority of cone bipolar cells showed sprouting, while horizontal cells maintained contacts with cones and cone-to-horizontal cell input was preserved. Furthermore a reduction of basal Ca²⁺ influx by a calcium channel blocker was not sufficient to rescue synaptic transmission deficits caused by the Ca_V1.4-IT mutation. Long term treatments with low-dose Ca²⁺ channel blockers might however be beneficial reducing Ca²⁺ toxicity without major effects on ganglion cells responses.

Sensory Processing at Ribbon Synapses in the Retina and the Cochlea

In recent years, sensory neuroscientists have made major efforts to dissect the structure and function of ribbon synapses which process sensory information in the eye and ear. This review aims to summarize our current understanding of two key aspects of ribbon synapses: 1) their mechanisms of exocytosis and endocytosis and 2) their molecular anatomy and physiology. Our comparison of ribbon synapses in the cochlea and the retina reveals convergent signaling mechanisms, as well as divergent strategies in different sensory systems.