Tran T S, Phelps P E
Department of Physiological Science, UCLA, Los Angeles, CA 90095-1527, USA.
Dev Neurosci. 2000;22(3):228-36. doi: 10.1159/000017445.
The neural cell adhesion molecule, L1, is thought to play a critical role in the formation and fasciculation of axon tracts during development. In the chick, the L1 cell adhesion molecule is expressed on both ipsi- and contralateral portions of commissural axons and perturbation studies produced a defasciculation of the ipsilateral commissural fibers. Yet in the rat, L1 is reported along commissural axons only after they have reached the contralateral marginal zone. When this species variation was reexamined, L1 was found to be expressed on rat commissural axons in a pattern similar to that observed in the chick. In addition, L1 is detected along commissural axons as early as embryonic day 12 in rats and maintained on both the ipsi- and contralateral surfaces during embryonic development. Other molecular markers that identify commissural axons in rats are TAG-1 (transiently expressed axonal glycoprotein) and DCC (deleted in colorectal cancer), and thus the pattern of L1 staining was compared with that of these other members of the immunoglobulin superfamily. Commissural axons emerging from dorsally located neurons are identified with TAG-1 and DCC, whereas L1 is detected only on ventrally located commissural axons. The pattern of L1 expression overlaps that of the more numerous laterally and ventromedially located GABAergic commissural axons. Furthermore, some of the GABAergic commissural axons express L1 on their surfaces. While commissural axons are often considered as a single population, differences in the combination of adhesion-type molecules on their surfaces and in their neurotransmitter phenotypes may signify distinctive neuronal subgroups.
神经细胞黏附分子L1被认为在发育过程中轴突束的形成和束状化中起关键作用。在鸡中，L1细胞黏附分子在连合轴突的同侧和对侧部分均有表达，而干扰研究导致同侧连合纤维出现束状分离。然而在大鼠中，据报道L1仅在连合轴突到达对侧边缘区后才沿着连合轴突出现。当重新审视这种物种差异时，发现L1在大鼠连合轴突上的表达模式与在鸡中观察到的相似。此外，早在大鼠胚胎第12天就能在连合轴突上检测到L1，并且在胚胎发育期间在同侧和对侧表面均持续存在。在大鼠中识别连合轴突的其他分子标记是TAG-1（瞬时表达的轴突糖蛋白）和DCC（结直肠癌缺失基因），因此将L1染色模式与免疫球蛋白超家族的这些其他成员的染色模式进行了比较。从背侧神经元发出的连合轴突可通过TAG-1和DCC识别，而L1仅在腹侧连合轴突上检测到。L1的表达模式与数量更多的位于外侧和腹内侧的GABA能连合轴突的表达模式重叠。此外，一些GABA能连合轴突在其表面表达L1。虽然连合轴突通常被视为单一群体，但它们表面上黏附型分子组合及其神经递质表型的差异可能表明存在不同的神经元亚群。
