Electrical synapses in rod and cone pathways of the mouse retina

哺乳动物的视网膜能够编码超过10个对数单位的光强度的视觉信息。因此，它进化出了两种类型的光感受器：用于弱光视觉的杆状感光器和用于强光和色觉的视锥。由连接蛋白构建的电突触(缝隙连接)在最敏感的视杆通路、初级视杆通路和视锥通路中起着至关重要的作用。在初级视杆通路中，微弱的光信号从视杆传递到视杆双极细胞，后者又与所有无长突细胞接触。所有细胞彼此之间形成同源细胞缝隙连接，从而在光子稀少时优化信噪比。AII细胞通过甘氨酸能突触向锥体外双极细胞传递视杆细胞信号，通过异质细胞缝隙连接向锥体外双极细胞传递视杆细胞信号。有趣的是，在光适应过程中，这些电突触的整顿变得更弱，有利于在明亮的光下从ON双极细胞到AII无长突细胞的信号流。AII-AII缝隙连接在超微结构和对神经调节剂的敏感性上与AII-On锥体双极细胞缝隙连接不同，尽管这些差异的结构基础只被部分了解。虽然人们认为AII-AII缝隙连接是由连接蛋白36(Cx36)组成的，但AII-On锥体双极细胞缝隙连接的组成和调控仍存在争议。我们现在有证据表明，所有细胞不仅表达Cx36，而且还表达另一种连接蛋白。我们假设这种连接蛋白参与了异质细胞AII缝隙连接，并使这些连接对调节剂敏感，这可能会以光依赖的方式诱导突触整流的变化。此外，我们最近发现AII-AII和AII-ON锥体双极细胞缝隙连接在组装机制上有所偏离，尽管这种差异的基础到目前为止还不清楚。因此，在这个项目中，我们的目标是确定同细胞和异细胞AII缝隙连接在结构、组装和光依赖调制方面的差异的分子基础。电突触对于锥体通路也是必不可少的。最近，我们与同事一起发现，在双极细胞上，不仅与AII无长突细胞相连，而且还与另一种甘氨酸能小野无长突细胞相连。这种连接是否是光依赖的，以及连接蛋白是连接蛋白的基础，到目前为止还不清楚。因此，我们的目标是分析连接蛋白偶联的调控，并确定参与其中的连接蛋白。为了实现我们的目标，我们将使用各种连接蛋白缺陷小鼠系和表达EGFP标记的连接蛋白的小鼠系进行免疫共沉淀、不同光照条件下的示踪剂偶联实验以及基于超分辨显微镜的定位分析。这将有助于理解电耦合的视网膜神经元如何能够与不同的突触伙伴建立电突触，以及这些突触如何根据环境光线水平进行不同的调制。

The mammalian retina is able to encode visual information over ~10 log units of light intensities. It therefore has evolved two types of photoreceptors: rods for vision under dim light and cones for vision under bright light and color vision. Electrical synapses (gap junctions) built from connexin proteins play an essential role in the most sensitive rod pathway, the primary rod pathway, and in cone pathways. In the primary rod pathway, dim light signals are mediated from rods to rod bipolar cells, which in turn contact AII amacrine cells. AII cells form homocellular gap junctions among each other, thereby optimizing signal-to-noise ratio when photons are scarce. AII cells send the rod signal via glycinergic synapses to OFF cone bipolar cells and via heterocellular gap junctions to ON cone bipolar cells. Interestingly, the rectification of these electrical synapses becomes weaker during light adaptation favoring signal flow from ON bipolar to AII amacrine cells in bright light.AII-AII gap junctions differ in ultrastructure and sensitivity to neuromodulators from AII-ON cone bipolar cell gap junctions although the structural basis for these differences is only partially understood. While it is believed that AII-AII gap junctions are composed of connexin36 (Cx36), composition and regulation of AII-ON cone bipolar cell gap junctions are still debated. We have evidence now that AII cells not only express Cx36 but also another connexin. We hypothesize that this connexin is involved in heterocellular AII gap junctions and makes these junctions sensitive to modulators, which may induce changes in synapse rectification in a light-dependent manner. Moreover, we recently found AII-AII and AII-ON cone bipolar cell gap junctions to deviate in their assembly mechanisms although the basis for this difference is not known so far. In this project, we thus aim to determine the molecular basis for the differences in structure, assembly, and light-dependent modulation of homo- and heterocellular AII gap junctions.Electrical synapses are also essential for cone pathways. Together with colleagues, we recently found that ON bipolar cells are not only coupled to the AII amacrine cell but also to another glycinergic small-field amacrine cell. Whether this coupling is light-dependent and which connexins underlie the coupling is not known so far. Thus, we aim to analyze the regulation of the coupling and to identify the connexins involved.To achieve our goals, various connexin-deficient mouse lines and mouse lines expressing EGFP-tagged connexins will be used for co-immunoprecipitation, tracer coupling experiments under different light conditions, and superresolution microscopy-based localization analyses. This will help to understand how electrically coupled retinal neurons are able to establish electrical synapses with different synaptic partners and how these synapses can be differentially modulated depending on ambient light levels.