The precise expression patterns of two IgLON genes, CEPU‐1 and limbic system‐associated membrane protein (LAMP), were studied during early embryogenesis. It was found that expression of both was localized to restricted regions of the brain and neural crest. In the developing neural tube, CEPU‐1 was expressed in the isthmus and a restricted region of the hindbrain, whereas LAMP was expressed in the anterior midbrain. Most neural crest cells expressed LAMP, whereas CEPU‐1 expression was limited to crest cells derived from the hindbrain. These results suggest that members of the IgLON family have important roles during embryogenesis, particularly in brain formation and differentiation.

In an attempt to isolate novel molecules involved in motoneuron differentiation and target muscle innervation during embryogenesis, we performed mRNA differential display analysis by comparing cDNAs of motoneurons purified by immunopanning from different portions along the rostro-caudal axis of chick embryonic spinal cord, and cloned an immunoglobulin superfamily protein named C30 (1). By sequence comparison, C30 was shown to be an alternatively spliced isoform of CEPU-1, which was formerly reported as a member of the immunoglobulin superfamily specifically expressed in cerebellar Purkinje cells (Spaltmann and Brummendorf, 1996, J. Neurosci. 16, 1770-1779). We analyzed the expression pattern of CEPU-1 both at the mRNA and protein levels in the spinal cord of the chick embryo. Until stage 23, CEPU-1 was expressed faintly in the ventral part of the neural tube but gradually it became localized to a specific group of cells. In the motor column, CEPU-1 was expressed transiently in many columnar layers. A C30-transfected cell line showed Ca2+-independent cell-cell binding activity. These results suggest a role for CEPU-1 in specific axon guidance and/or fasciculation of motoneurons during development. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.

To identify molecules that might be involved in the early development of motoneurons, we performed RT-PCR for receptor tyrosine kinases (RTKs) and mRNA differential display with embryonic day 5 (E5) chick motoneurons enriched by panning. In situ hybridization revealed that Cek88 RTK among the cloned molecules was specifically expressed on motoneurons at the brachial and lumbar segments of the spinal cord which innervate limb muscles. We found that the expression of Cek8 in limb motoneurons was already intrinsically determined at E2. To analyze how motoneurons acquire regional specification along the anterior-posterior (A-P) axis and in the transverse plane, we observed the expression of Cek8, Islet-1, SC1 and low-affinity NGF receptor on ectopic motoneurons induced by implanting a supernumerary notochord or floor plate. The ectopic motoneurons induced by the graft obtained from either the thoracic or lumbar segments had the same expression profile as the in situ motoneurons at each A-P level. These findings suggest that regional specification of motoneurons is independent of the notochord and the floor plate and that the whole neural tube is committed to differentiate into the motoneuron subtypes along the A-P axis at the operative stages.