Ha Seungshin, Tripathi Prem P, Mihalas Anca B, Hevner Robert F, Beier David R
Center for Developmental Biology and Regenerative Medicine and.
Division of Genetic Medicine, Department of Pediatrics, and.
J Neurosci. 2017 Jan 25;37(4):960-971. doi: 10.1523/JNEUROSCI.1826-16.2016.
We discovered a hypomorphic reelin (Reln) mutant with abnormal cortical lamination and no cerebellar hypoplasia. This mutant, Reln, carries a chemically induced splice-site mutation that truncates the C-terminal region (CTR) domain of RELN protein and displays remarkably distinct phenotypes from reeler The mutant does not have an inverted cortex, but cortical neurons overmigrate and invade the marginal zone, which are characteristics similar to a phenotype seen in the cerebral cortex of Vldlr mice. The dentate gyrus shows a novel phenotype: the infrapyramidal blade is absent, while the suprapyramidal blade is present and laminated. Genetic epistasis analysis showed that Reln/Apoer2 double homozygotes have phenotypes akin to those of reeler mutants, while Reln/Vldlr mice do not. Given that the receptor double knock-out mice resemble reeler mutants, we infer that Reln/Apoer2 double homozygotes have both receptor pathways disrupted. This suggests that CTR-truncation disrupts an interaction with VLDLR (very low-density lipoprotein receptor), while the APOER2 signaling pathway remains active, which accounts for the hypomorphic phenotype in Reln mice. A RELN-binding assay confirms that CTR truncation significantly decreases RELN binding to VLDLR, but not to APOER2. Together, the in vitro and in vivo results demonstrate that the CTR domain confers receptor-binding specificity of RELN.
Reelin signaling is important for brain development and is associated with human type II lissencephaly. Reln mutations in mice and humans are usually associated with cerebellar hypoplasia. A new Reln mutant with a truncation of the C-terminal region (CTR) domain shows that Reln mutation can cause abnormal phenotypes in the cortex and hippocampus without cerebellar hypoplasia. Genetic analysis suggested that CTR truncation disrupts an interaction with the RELN receptor VLDLR (very low-density lipoprotein receptor); this was confirmed by a RELN-binding assay. This result provides a mechanistic explanation for the hypomorphic phenotype of the CTR-deletion mutant, and further suggests that Reln mutations may cause more subtle forms of human brain malformation than classic lissencephalies.
我们发现了一种低表达的瑞连蛋白（Reln）突变体，其具有异常的皮质分层且无小脑发育不全。该突变体Reln携带化学诱导的剪接位点突变，该突变截断了RELN蛋白的C末端区域（CTR）结构域，并表现出与reeler明显不同的表型。该突变体没有倒置的皮质，但皮质神经元过度迁移并侵入边缘区，这些特征与Vldlr小鼠大脑皮质中所见的表型相似。齿状回表现出一种新的表型：锥体下叶片缺失，而锥体上叶片存在且分层。遗传上位性分析表明，Reln/Apoer2双纯合子具有与reeler突变体相似的表型，而Reln/Vldlr小鼠则没有。鉴于受体双敲除小鼠类似于reeler突变体，我们推断Reln/Apoer2双纯合子的两条受体途径均被破坏。这表明CTR截断破坏了与极低密度脂蛋白受体（VLDLR）的相互作用，而APOER2信号通路仍然活跃，这解释了Reln小鼠的低表达表型。一项RELN结合试验证实，CTR截断显著降低了RELN与VLDLR的结合，但不影响与APOER2的结合。总之，体外和体内结果表明，CTR结构域赋予了RELN受体结合特异性。
瑞连蛋白信号传导对大脑发育很重要，并且与人类II型无脑回畸形有关。小鼠和人类中的Reln突变通常与小脑发育不全有关。一种新的C末端区域（CTR）结构域截断的Reln突变体表明，Reln突变可导致皮质和海马体出现异常表型，而无小脑发育不全。遗传分析表明，CTR截断破坏了与RELN受体VLDLR（极低密度脂蛋白受体）的相互作用；这通过一项RELN结合试验得到了证实。这一结果为CTR缺失突变体的低表达表型提供了一个机制解释，并进一步表明，Reln突变可能导致比经典无脑回畸形更微妙的人类脑畸形形式。
