Needham L K, Thelen K, Maness P F
Department of Biochemistry, School of Medicine, University of North Carolina, Chapel Hill, North Carolina 27599-7260, USA.
J Neurosci. 2001 Mar 1;21(5):1490-500. doi: 10.1523/JNEUROSCI.21-05-01490.2001.
The neural adhesion molecule L1 mediates the axon outgrowth, adhesion, and fasciculation that are necessary for proper development of synaptic connections. L1 gene mutations are present in humans with the X-linked mental retardation syndrome CRASH (corpus callosum hypoplasia, retardation, aphasia, spastic paraplegia, hydrocephalus). Three missense mutations associated with CRASH syndrome reside in the cytoplasmic domain of L1, which contains a highly conserved binding region for the cytoskeletal protein ankyrin. In a cellular ankyrin recruitment assay that uses transfected human embryonic kidney (HEK) 293 cells, two of the pathologic mutations located within the conserved SFIGQY sequence (S1224L and Y1229H) strikingly reduced the ability of L1 to recruit 270 kDa ankyrinG protein that was tagged with green fluorescent protein (ankyrin-GFP) to the plasma membrane. In contrast, the L1 missense mutation S1194L and an L1 isoform lacking the neuron-specific sequence RSLE in the cytoplasmic domain were as effective as RSLE-containing neuronal L1 in the recruitment of ankyrin-GFP. Ankyrin binding by L1 was independent of cell-cell interactions. Receptor-mediated endocytosis of L1 regulates intracellular signal transduction, which is necessary for neurite outgrowth. In rat B35 neuroblastoma cell lines stably expressing L1 missense mutants, antibody-induced endocytosis was unaffected by S1224L or S1194L mutations but appeared to be enhanced by the Y1229H mutation. These results suggested a critical role for tyrosine residue 1229 in the regulation of L1 endocytosis. In conclusion, specific mutations within key residues of the cytoplasmic domain of L1 (Ser(1224), Tyr(1229)) destabilize normal L1-ankyrin interactions and may influence L1 endocytosis to contribute to the mechanism of neuronal dysfunction in human X-linked mental retardation.
神经黏附分子L1介导轴突生长、黏附和成束，这些过程对于突触连接的正常发育是必需的。L1基因突变存在于患有X连锁智力迟钝综合征CRASH（胼胝体发育不全、智力迟钝、失语、痉挛性截瘫、脑积水）的人类中。与CRASH综合征相关的三个错义突变位于L1的细胞质结构域，该结构域包含一个与细胞骨架蛋白锚蛋白高度保守的结合区域。在使用转染的人胚肾（HEK）293细胞的细胞锚蛋白招募试验中，位于保守的SFIGQY序列内的两个病理突变（S1224L和Y1229H）显著降低了L1将标记有绿色荧光蛋白的270 kDa锚蛋白G蛋白（锚蛋白-GFP）招募到质膜的能力。相比之下，L1错义突变S1194L和细胞质结构域中缺乏神经元特异性序列RSLE的L1同工型在招募锚蛋白-GFP方面与含有RSLE的神经元L1一样有效。L1与锚蛋白的结合独立于细胞间相互作用。L1的受体介导的内吞作用调节细胞内信号转导，这对于神经突生长是必需的。在稳定表达L1错义突变体的大鼠B35神经母细胞瘤细胞系中，抗体诱导的内吞作用不受S1224L或S1194L突变的影响，但似乎因Y1229H突变而增强。这些结果表明酪氨酸残基1229在L1内吞作用的调节中起关键作用。总之，L1细胞质结构域关键残基内的特定突变（Ser(1224)，Tyr(1229)）破坏了正常的L1-锚蛋白相互作用，并可能影响L1内吞作用，从而导致人类X连锁智力迟钝中神经元功能障碍的机制。
