Cell adhesion proteins of the Cadm (SynCAM/Necl) family regulate myelination and the organization of myelinated axons. In the peripheral nervous system (PNS), intercellular contact between Schwann cells and their underlying axons is believed to be mediated by binding of glial Cadm4 to axonal Cadm3 or Cadm2. Nevertheless, given that distinct neurons express different combinations of the Cadm proteins, the identity of the functional axonal ligand for Cadm4 remains to be determined. Here, we took a genetic approach to compare the phenotype of Cadm4 null mice, which exhibit abnormal distribution of Caspr and Kv1 potassium channels, with mice lacking different combinations of Cadm1–Cadm3 genes. We show that in contrast to mice lacking the single Cadm1, Cadm2, or Cadm3 genes, genetic ablation of all three phenocopies the abnormalities detected in the absence of Cadm4. Similar defects were observed in double mutant mice lacking Cadm3 and Cadm2 (i.e., Cadm3^–/–/Cadm2^–/–) or Cadm3 and Cadm1 (i.e., Cadm3^–/–/Cadm1^–/–), but not in mice lacking Cadm1 and Cadm2 (i.e., Cadm1^–/–/Cadm2^–/–). Furthermore, axonal organization abnormalities were also detected in Cadm3 null mice that were heterozygous for the two other axonal Cadms. Our results identify Cadm3 as the main axonal ligand for glial Cadm4, and reveal that its absence could be compensated by the combined action of Cadm2 and Cadm1.

SIGNIFICANCE STATEMENT Myelination by Schwann cells enables fast conduction of action potentials along motor and sensory axons. In these nerves, Schwann cell-axon contact is mediated by cell adhesion molecules of the Cadm family. Cadm4 in Schwann cells regulates axonal ensheathment and myelin wrapping, as well as the organization of the axonal membrane, but the identity of its axonal ligands is not clear. Here, we reveal that Cadm mediated axon-glia interactions depend on a hierarchical adhesion code that involves multiple family members. Our results provide important insights into the molecular mechanisms of axon-glia communication, and the function of Cadm proteins in PNS myelin.

Cadm (SynCAM/Necl) 家族的细胞粘附蛋白调节髓鞘形成和有髓鞘轴突的组织。在外周神经系统 (PNS) 中，雪旺氏细胞与其潜在轴突之间的细胞间接触被认为是通过神经胶质 Cadm4 与轴突 Cadm3 或 Cadm2 的结合介导的。然而，鉴于不同的神经元表达不同的 Cadm 蛋白组合，Cadm4 的功能性轴突配体的身份仍有待确定。在这里，我们采用遗传方法来比较Cadm4缺失小鼠的表型，这些小鼠表现出 Caspr 和 Kv1 钾通道的异常分布，与缺乏Cadm1 – Cadm3基因不同组合的小鼠。我们表明，与缺乏单个Cadm1 的小鼠相比、Cadm2或Cadm3基因，所有三个表型的基因消融在不存在 Cadm4 的情况下检测到异常。在缺乏 Cadm3 和 Cadm2（即Cadm3 ^–/– /Cadm2 ^–/–）或 Cadm3 和 Cadm1（即Cadm3 ^–/– /Cadm1 ^–/–）的双突变小鼠中观察到类似的缺陷，但在缺乏 Cadm1 和Cadm2（即Cadm1 ^–/– /Cadm2 ^–/–）。此外，在Cadm3中也检测到轴突组织异常对另外两个轴突 Cadms 杂合的空小鼠。我们的结果确定 Cadm3 是神经胶质 Cadm4 的主要轴突配体，并表明 Cadm2 和 Cadm1 的联合作用可以补偿它的缺失。

意义声明雪旺氏细胞的髓鞘形成使动作电位沿运动和感觉轴突快速传导。在这些神经中，雪旺细胞-轴突接触由 Cadm 家族的细胞粘附分子介导。Schwann 细胞中的 Cadm4 调节轴突包裹和髓鞘包裹，以及轴突膜的组织，但其轴突配体的身份尚不清楚。在这里，我们揭示了 Cadm 介导的轴突-胶质细胞相互作用取决于涉及多个家庭成员的分层粘附代码。我们的研究结果为轴突-胶质细胞通讯的分子机制以及 Cadm 蛋白在 PNS 髓鞘中的功能提供了重要的见解。