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Axons crossing in the ventral commissure express L1 and GAD65 in the developing rat spinal cord
1Department of Physiological Science, UCLA, Los Angeles, CA 90095-1527, USA.
Insights
Neural cell adhesion molecule L1 is crucial for axon tract formation. Re-examination reveals L1 expression on rat commissural axons, similar to chicks, aiding in understanding neuronal development.
Area of Science:
- Neuroscience
- Developmental Biology
- Cell Adhesion
Background:
- Neural cell adhesion molecule L1 (L1) is implicated in axon tract formation and fasciculation.
- Previous studies suggested species-specific expression patterns of L1 in developing commissural axons.
Purpose of the Study:
- To re-examine and clarify the expression pattern of L1 on rat commissural axons during embryonic development.
- To compare L1 expression with other molecular markers like TAG-1 and DCC in rats.
Main Methods:
- Immunohistochemical analysis of L1 expression on embryonic rat brain sections.
- Comparative analysis of L1 staining with TAG-1 and DCC markers.
Main Results:
- L1 is expressed on both ipsi- and contralateral portions of rat commissural axons, contrary to previous reports.
- L1 expression is detected as early as embryonic day 12 in rats and persists throughout development.
- L1 expression pattern on rat commissural axons resembles that observed in chick embryos.
- L1 is found on ventrally located commissural axons, overlapping with GABAergic axons, while TAG-1 and DCC mark dorsally located ones.
Conclusions:
- The expression pattern of L1 in rat commissural axons is conserved with chick, playing a role in axon guidance.
- Distinct combinations of adhesion molecules and neurotransmitter phenotypes suggest subgroups within commissural axons.
Abstract:
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.