Department of Biochemistry and Molecular Genetics, University of Louisville, Louisville, KY 40292.
Department of Ophthalmology and Visual Sciences, University of Louisville, Louisville, KY 40292.
eNeuro. 2020 Feb 11;7(1). doi: 10.1523/ENEURO.0002-20.2020. Print 2020 Jan/Feb.
The first retinal synapse, photoreceptor→bipolar cell (BC), is both anatomically and functionally complex. Within the same synaptic region, a change in presynaptic glutamate release is sensed by both ON BCs (DBCs) via the metabotropic glutamate receptor 6 (mGluR6), and OFF BCs (HBCs) via ionotropic glutamate receptors to establish parallel signaling pathways that preferentially encode light increments (ON) or decrements (OFF), respectively. The synaptic structural organization of ON and OFF-type BCs at the photoreceptor terminal differs. DBCs make an invaginating synapse that contains a diverse but incompletely understood complex of interacting proteins (signalplex). HBCs make primarily flat contacts that contain an apparent different set of proteins that is equally uncharacterized. LRIT3 is a synaptic protein known to be essential for ON pathway visual function. In both male and female mice, we demonstrate that LRIT3 interacts with and is required for expression of nyctalopin, and thus TRPM1 at all DBC dendritic tips, but DBC signalplex components are not required for LRIT3 expression. Using whole-cell and multielectrode array (MEA) electrophysiology and glutamate imaging, we demonstrate that the loss of LRIT3 impacts both ON and OFF signaling pathway function. Without LRIT3, excitatory input to type 1 BCs is reduced, as are the visually evoked responses of many OFF retinal ganglion cells (RGCs). We conclude that the absence of LRIT3 expression disrupts excitatory input to OFF BCs and, thus disrupts the normal function of OFF RGCs.
光感受器→双极细胞（BC）的第一个视网膜突触在解剖和功能上都非常复杂。在同一突触区域内，ON BC（DBC）通过代谢型谷氨酸受体 6（mGluR6）和 OFF BC（HBC）通过离子型谷氨酸受体感知到突触前谷氨酸释放的变化，从而建立平行的信号通路，分别优先编码光的增加（ON）或减少（OFF）。ON 和 OFF 型 BC 在光感受器末端的突触结构组织不同。DBC 形成一个内陷的突触，其中包含多种但不完全了解的相互作用蛋白（信号复合物）。HBC 形成主要的平面接触，其中包含一组明显不同的蛋白质，同样也未被描述。LRIT3 是一种已知对 ON 通路视觉功能至关重要的突触蛋白。在雄性和雌性小鼠中，我们证明 LRIT3 与 nyctalopin 相互作用并需要其表达，从而在所有 DBC 树突尖端表达 TRPM1，但 DBC 信号复合物成分不是 LRIT3 表达所必需的。使用全细胞膜片钳和多电极阵列（MEA）电生理学和谷氨酸成像，我们证明 LRIT3 的缺失会影响 ON 和 OFF 信号通路的功能。没有 LRIT3，1 型 BC 的兴奋性输入减少，许多 OFF 视网膜神经节细胞（RGC）的视觉诱发反应也减少。我们得出结论，LRIT3 表达的缺失会破坏 OFF BC 的兴奋性输入，从而破坏 OFF RGC 的正常功能。
