Department of Biology, Hyogo College of Medicine, Mukogawa, Nishinomiya, Hyogo, Japan.
Studio EM-Retina, Satonaka, Nishinomiya, Hyogo, Japan.
Invest Ophthalmol Vis Sci. 2021 Jan 4;62(1):31. doi: 10.1167/iovs.62.1.31.
The three-dimensional configurations of rod and cone bipolar cell (BC) dendrites and horizontal cell (HC) processes outside rod and cone synaptic terminals have not been fully elucidated. We reveal how these neurites are mutually arranged to coordinate formation and maintenance of the postsynaptic complex of ribbon synapses in mouse and monkey retinas.
Serial section transmission electron microscopy was utilized to reconstruct BC and HC neurites in macaque monkey and mouse, including metabotropic glutamate receptor 6 (mGluR6)-knockout mice.
Starting from sporadically distributed branching points, rod BC and HC neurites (B and H, respectively) took specific paths to rod spherules by gradually adjusting their mutual positions, which resulted in a closed alternating pattern of H‒B‒H‒B neurites at the rod spherule aperture. This order corresponded to the array of elements constituting the postsynaptic complex of ribbon synapses. We identified novel helical coils of HC processes surrounding the rod BC dendrite in both mouse and macaque retinas, and these structures occurred more frequently in mGluR6-knockout than wild-type mouse retinas. Horizontal cell processes also formed hook-like protrusions that encircled cone BC and HC neurites below the cone pedicles in the macaque retina.
Bipolar and horizontal cell neurites take specific paths to adjust their mutual positions at the rod spherule aperture. Some HC processes are helically coiled around rod BC dendrites or form hook-like protrusions around cone BC dendrites and HC processes. Loss of mGluR6 signaling may be one factor promoting unbalanced neurite growth and compensatory neurite coiling.
杆状和锥状双极细胞（BC）树突和水平细胞（HC）突起在杆状和锥状突触末端外的三维构型尚未完全阐明。我们揭示了这些神经突是如何相互排列的，以协调在小鼠和猴视网膜中的带状突触的突触后复合物的形成和维持。
利用连续切片透射电子显微镜重建了猕猴和小鼠（包括代谢型谷氨酸受体 6（mGluR6）-敲除小鼠）的 BC 和 HC 神经突。
从分散的分支点开始，杆状 BC 和 HC 神经突（分别为 B 和 H）通过逐渐调整它们的相互位置，采取特定的路径到达杆状小球，导致在杆状小球开口处 H-B-H-B 神经突的封闭交替模式。这种顺序与构成带状突触的突触后复合物的元素排列相对应。我们在小鼠和猕猴的视网膜中都发现了 HC 突起围绕杆状 BC 树突的新颖的螺旋线圈，并且这些结构在 mGluR6 敲除小鼠的视网膜中比野生型小鼠的视网膜中更频繁地发生。水平细胞突起也形成钩状突起，围绕在锥体小球下方的锥体 BC 和 HC 神经突周围。
双极和水平细胞神经突采取特定的路径来调整它们在杆状小球开口处的相互位置。一些 HC 突起围绕杆状 BC 树突螺旋缠绕，或围绕锥体 BC 树突和 HC 突起形成钩状突起。mGluR6 信号的缺失可能是促进神经突生长不平衡和代偿性神经突缠绕的一个因素。
