Molecular mechanisms of columnar unit formation by differential cell adhesion in the brain

柱状结构是大脑的基本单位，但柱状结构的形成机制仍不清楚。在本研究中，我们提出了基于果蝇脑内必需柱状神经元之间粘附水平差异的柱状组织机制，由细胞粘附分子N-cadherin （N-cad）和Flamingo （Fmi）调节。我们发现柱的形成是由三个核心神经元R8、R7和Mi1发起的，并证明这些柱状神经元的ncad依赖的相对粘附性根据差异粘附假说（DAH）调节它们在柱内的相对位置。首先，我们检查了这些神经元中Ncad的水平，发现柱状神经元含有更多的Ncad，按R7 > R8 > Mi1的顺序排列。然后，我们降低粘附性最强的R7神经元的Ncad水平，发现R7从柱的中心向柱的周围区域扩展。当R8神经元中Ncad水平降低时，观察到类似的表型：这些神经元的末端扩大并重定向到柱外的外周区域。相反，在粘附性较差的Mi1神经元中，Ncad的过表达诱导这些神经元向柱中心重定向。Ncad在R8神经元中的过表达导致R8末梢向柱中心广泛投射。此外，我们建立了一个数学模型，证实了核心柱状神经元的排列可以完全通过这些核心柱状神经元之间的差异粘附来实现。

Columnar structure is a basic unit of the brain, but the mechanism of column formation remains largely unknown. In this study, we proposed the mechanism of column organization based on the differential level of adhesion between essential columnar neurons in the fly brain, regulated by cell adhesion molecules N-cadherin (N-cad) and Flamingo (Fmi). We revealed that column formation is initiated by three core neurons, R8, R7, and Mi1, and demonstrated that Ncad-dependent relative adhesiveness of these columnar neurons regulates their relative location within a column according to the differential adhesion hypothesis (DAH). First, we examined the level of Ncad in each of these neurons and found that columnar neurons contain more Ncad in the order of R7 > R8 > Mi1. Then, we decreased the level of Ncad in the most adhesive R7 neurons and found that the R7 expands from the center of the column to the peripheral area of the column. When Ncad level was reduced in R8 neurons, similar phenotype was observed: terminals of these neurons expanded and redirected to the peripheral area outside the column. In the contrary, overexpression of Ncad in less adhesive Mi1 neurons induced the redirection of these neurons toward the column center. Ncad overexpression in R8 neurons led to the extensive projection of R8 terminals to the center of the column. In addition, we established a mathematical model that confirms that the arrangement of core columnar neurons can be achieved solely by the differential adhesion between these core columnar neurons.