Molecular control of neuronal shape and connectivity in the developing retina

 描述（由申请人提供）：神经元表现出不同的形态，影响信息如何通过复杂的连接网络传播和调制。电路功能的一个关键决定因素是树突的数量和排列。例如，局部中间神经元从细胞体对称地延伸出多个树突，而小脑浦肯野神经元精心制作了一个局限于分子层的巨大树突。与轴突一样，树突也是从分化中的神经元的细胞体延伸出来的全能神经突发育而来。一个神经突变成了轴突。剩下的神经突要么缩回，要么保留下来发育成树突。在体内，这些事件与周围组织协调，使得轴突和树突在定型位置发育，在那里它们被完美地定位以与适当的突触伴侣相互作用。我们的长期目标是 了解外在信号如何改变幼稚神经突的内在特性，从而确保神经元获得与电路其余部分正确定向的极化形态。为了解决这个问题，我们将研究无长突细胞中树突特化的机制，该细胞调节从外部到内部视网膜的信息流。无长突细胞发育成一个单一的初级树突，指向一个被称为内丛状层（IPL）的神经节的限定区域。发育中的无长突细胞在迁移时是双极的，但在接触新生IPL时变成单极：接触IPL的神经突保留为树突，但细胞另一极上的神经突缩回。我们开发了一种延时成像系统，使我们能够在视网膜中记录无长突细胞从双极形态转变为单极形态时，无论是在整体细胞形状水平还是在细胞骨架水平。我们发现，这种极性变化的一个关键读数是高尔基体的位置，它移动到新生的初级树突。在小鼠突变的非典型钙粘蛋白脂肪3，这种转变并不可靠地发生，导致出现无长突细胞与两个树突状乔木和异位放置的高尔基体。作为一种具有保守的胞内结构域的跨膜受体，Fat 3为理解外在线索如何导致神经元形态的内在变化提供了一个有力的切入点。事实上，Fat 3细胞内结构域不仅与已知的肌动蛋白调节因子（即Ena/VASP家族成员）结合，而且与控制微管动力学的蛋白质（即CLASP 1/2）结合，表明Fat 3协调肌动蛋白和微管的双重作用。通过将Fat 3及其效应物的分子和遗传研究与原位无长突细胞树突发育的时间推移分析相结合，我们将获得对支配树突特化的外在和内在机制的新见解。

 DESCRIPTION (provided by applicant): Neurons exhibit diverse morphologies that influence how information is propagated and modulated through complex networks of connections. One key determinant of circuit function is the number and arrangement of dendrites. For instance, local interneurons extend multiple dendrites symmetrically from the cell body, whereas cerebellar Purkinje neurons elaborate a single huge dendritic arbor that is confined to the molecular layer. Like axons, dendrites develop from totipotent neurites that extend from the cell body of the differentiating neuron. One neurite becomes an axon. The remaining neurites are either retracted or retained to develop as dendrites. In vivo, these events are coordinated with the surrounding tissue, such that axons and dendrites develop in stereotyped locations where they are perfectly positioned to interact with appropriate synaptic partners. Our long term goal is to understand how extrinsic signals alter the intrinsic properties of naïve neurites, thereby ensuring that neurons acquire polarized morphologies that are correctly oriented with the rest of the circuit. To tackle this question, we will investigate mechanisms of dendrite specification in amacrine cells, which modulate the flow of information from the outer to the inner retina. Amacrine cells develop a single primary dendrite that points into a defined region of neuropil called the inner plexiform layer (IPL). Developing amacrine cells are bipolar as they migrate but become unipolar upon contacting the nascent IPL: the neurite that contacts the IPL is retained as a dendrite, but the neurite on the opposite pole of the cell is retracted. We have developed a time‐lapse imaging system that allows us to document amacrine cells in the retina as they transition from a bipolar to unipolar morphology, both at the level of the overall cell shape and a the level of the cytoskeleton. We find that a key readout for this change in polarity is the positin of the Golgi apparatus, which moves into the nascent primary dendrite. In mice mutant for the atypical cadherin Fat3, this transition does not occur reliably, leading to the appearance of amacrine cells with two dendritic arbors and an ectopically placed Golgi apparatus. As a transmembrane receptor with a conserved intracellular domain harboring protein‐binding motifs, Fat3 offers a potent entry point for understanding how extrinsic cues lead to intrinsic changes in neuronal morphology. Indeed, the Fat3 intracellular domain binds not only to known actin regulators (i.e. Ena/VASP family members) but also to proteins that control microtubule dynamics (i.e. CLASP1/2), suggesting that Fat3 coordinates dual effects on actin and microtubules. By pairing molecular and genetic studies of Fat3 and its effectors with time‐lapse analysis of amacrine cell dendrite development in situ, we will gain new insights into the extrinsic and intrinsic mechanisms that govern dendrite specification.