Development of the blue cone bipolar cell in the mouse retina

神经元过程如何发展并与突触伙伴建立适当的联系是神经科学的基本问题之一。脊椎动物视网膜是研究树突发育和神经元连接的优秀模型系统。色觉是重要的视觉功能之一，它需要视网膜神经元的精确连接。在小鼠视网膜中有两种视锥感受器，一种是仅表达S视蛋白的短波敏视锥(S视锥)，另一种是长波敏视锥(M视锥)，其中许多视锥共表达S视蛋白。为了产生颜色对抗性，来自这两种锥体的信号必须在神经节细胞水平上进行对比之前被分离。S球果仅占总球果种群的2-5%。因此，下游的S锥双极细胞(SCBC)面临着从大多数M锥中寻找非常稀疏的S锥的艰巨任务。其结果是，SCBC形成了一种非常独特的树枝状乔木，树枝上有细长的树枝，与少数S球果接触。S锥体和SCBC之间的这种独特的联系使其成为研究突触前神经元如何影响突触后神经元的树突发育和突触靶向的一个很好的模型系统。 我们利用了一个动物模型(Thrb2-/-小鼠)，在该模型中，M-opsin的表达被取消，所有的M-视锥细胞都变成了蓝色视锥细胞，并询问这种锥体传入细胞数量和类型的变化将如何影响SCBC的树突发育和突触形成。我们获得了CLM/Thrb2+/+、CLM/Thrb2+/-和CLM/Thrb2-/-小鼠，并比较了这些小鼠的SCBC的树突状形态。我们发现Thrb2-/-小鼠的SCBC数量与野生型和Thrb2+/-小鼠相当。在形态上，Thrb2-/-小鼠的SCBC与野生型和Thrb2+/-小鼠的SCBC在树突长度、树突分支数量和锥体接触数量方面没有区别。我们的结果表明，SCBC树突状细胞的发育似乎不依赖于S视蛋白表达锥的传入输入，而真正的S视锥可能受控制S视蛋白表达的Trb2以外的其他转录因子的特异性。

How neuronal processes develop and establish proper wirings with their synaptic partners is one of the fundamental questions of neuroscience. The vertebrate retina is an outstanding model system for studying dendritic development and neuronal connections. One of the critical visual functions, color vision, requires precise wiring of retinal neurons. In the mouse retina, there are two types of cone photoreceptors, the short wavelength sensitive cones (S-cones), which only express S-opsin, and the long wavelength sensitive cones (M-cones), many of which co-express S-opsin. In order to generate color opponency, signals from these two types of cones have to be segregated before they are contrasted at the ganglion cell level. S-cones only account for 2-5% of the total cone population. Thus, the downstream S-cone bipolar cells (SCBCs) face the daunting task of seeking out very sparse S-cones from a majority of M-cones. The outcome is that SCBCs develop a very unique dendritic arbor with long, meager dendrites that contact a handful of S-cones. This distinctive connection between S-cones and SCBCs makes it an excellent model system to study how presynaptic neurons affect the dendritic development and synaptic targeting of postsynaptic neurons. We took advantage of an animal model (Thrb2-/- mice), where M-opsin expression is abolished and all M-cones are turned into blue cones and asked how this alteration in number and type of cone afferents will affect the dendritic development and synapse formation of SCBCs. We obtained Clm/Thrb2+/+, Clm/ Thrb2+/- and Clm/Thrb2-/- mice and compared dendritic morphology of SCBCs from these mice. We found that the number of SCBCs from Thrb2-/- mice is comparable to that in wildtype and Thrb2+/- mice. Morphologically, SCBCs from Thrb2-/- mice are indistinguishable from those in wildtype and Thrb2+/- mice in terms of length of dendrites, number of dendritic branches and number of cone contacts. Our results indicate that dendritic development of SCBCs appears to be independent of the afferent input from S-opsin expressing cones, and that true S-cones may be specified by other transcription factors than Trb2, which controls S-opsin expression.