Unidirectional Photoreceptor-to-Müller Glia Coupling and Unique K+ Channel Expression in Caiman Retina

BackgroundMüller cells, the principal glial cells of the vertebrate retina, are fundamental for the maintenance and function of neuronal cells. In most vertebrates, including humans, Müller cells abundantly express Kir4.1 inwardly rectifying potassium channels responsible for hyperpolarized membrane potential and for various vital functions such as potassium buffering and glutamate clearance; inter-species differences in Kir4.1 expression were, however, observed. Localization and function of potassium channels in Müller cells from the retina of crocodiles remain, hitherto, unknown.MethodsWe studied retinae of the Spectacled caiman (Caiman crocodilus fuscus), endowed with both diurnal and nocturnal vision, by (i) immunohistochemistry, (ii) whole-cell voltage-clamp, and (iii) fluorescent dye tracing to investigate K+ channel distribution and glia-to-neuron communications.ResultsImmunohistochemistry revealed that caiman Müller cells, similarly to other vertebrates, express vimentin, GFAP, S100β, and glutamine synthetase. In contrast, Kir4.1 channel protein was not found in Müller cells but was localized in photoreceptor cells. Instead, 2P-domain TASK-1 channels were expressed in Müller cells. Electrophysiological properties of enzymatically dissociated Müller cells without photoreceptors and isolated Müller cells with adhering photoreceptors were significantly different. This suggests ion coupling between Müller cells and photoreceptors in the caiman retina. Sulforhodamine-B injected into cones permeated to adhering Müller cells thus revealing a uni-directional dye coupling.ConclusionOur data indicate that caiman Müller glial cells are unique among vertebrates studied so far by predominantly expressing TASK-1 rather than Kir4.1 K+ channels and by bi-directional ion and uni-directional dye coupling to photoreceptor cells. This coupling may play an important role in specific glia-neuron signaling pathways and in a new type of K+ buffering.研究背景：米勒胶质细胞（Müller cells）是脊椎动物视网膜的主要胶质细胞，对神经元细胞的维持与功能发挥至关重要。在包括人类在内的多数脊椎动物中，米勒胶质细胞会大量表达Kir4.1内向整流钾离子通道（Kir4.1 inwardly rectifying potassium channels），该通道负责维持细胞膜超极化电位，并参与钾离子缓冲、谷氨酸清除等多种关键生理功能；但已有研究显示不同物种间的Kir4.1表达存在差异。迄今为止，鳄鱼视网膜内米勒胶质细胞上钾离子通道的定位与功能仍未明确。 研究方法：本研究以兼具昼行与夜行视觉特性的眼镜凯门鳄（Spectacled caiman, Caiman crocodilus fuscus）的视网膜为研究对象，通过以下三种实验手段探究钾离子通道的分布模式以及胶质细胞-神经元信号交流机制：(1) 免疫组织化学染色；(2) 全细胞膜片钳技术；(3) 荧光染料示踪法。 实验结果：免疫组织化学结果显示，与其他脊椎动物类似，凯门鳄的米勒胶质细胞表达波形蛋白（vimentin）、胶质纤维酸性蛋白（GFAP）、S100β蛋白以及谷氨酰胺合成酶。与之相反，米勒胶质细胞中未检测到Kir4.1通道蛋白，该蛋白仅定位于感光细胞；而米勒胶质细胞表达的是双孔结构域TASK-1通道（2P-domain TASK-1 channels）。经酶解分离的无感光细胞附着的米勒胶质细胞，与保留粘附感光细胞的分离米勒胶质细胞，其电生理特性存在显著差异，这表明凯门鳄视网膜内米勒胶质细胞与感光细胞之间存在离子偶联。将磺酰罗丹明B（Sulforhodamine-B）注入视锥细胞后，该染料可渗透至粘附的米勒胶质细胞，证实二者之间存在单向染料偶联。 研究结论：本研究数据表明，在目前已被研究的脊椎动物中，凯门鳄米勒胶质细胞具有独特性：其主要表达TASK-1钾离子通道而非Kir4.1通道，且与感光细胞之间存在双向离子偶联与单向染料偶联。这种偶联机制可能在特定的胶质细胞-神经元信号通路以及新型钾离子缓冲机制中发挥重要作用。