Catalytic and non-catalytic forms of the neurotrophin receptor xTrkB mRNA are expressed in a pseudo-segmental manner within the early Xenopus central nervous system.

The induction of anterior-posterior and medio-lateral patterning within the Xenopus neural plate leads to the rapid establishment of a functional nervous system. Here we describe two Xenopus TrkB neurotrophin receptor genes which are expressed in discrete sets of neuroblasts during this developmental process. The xTrkB mRNAs encode both catalytic and non-catalytic receptors and exhibit membrane-spanning-domain proximal splicing. Expression begins at neural tube closure within the trigeminal ganglion and within the Rohon-Beard neurons of the dorsal spinal cord, providing an excellent dorsal marker of early neural tube patterning. Expression occurs later in the facial ganglia and possibly within the Kolmer-Agduhr neurons. The predominant xTrkB transcripts within the trigeminal and Rohon-Beard neurons and the exclusive early transcripts of the facial ganglia encode C-terminally truncated non-catalytic receptors. Such Trk mRNAs have previously been observed in rodents. However, our observations suggest that they may play a specific role during early development. Anterior-posterior segmentation of the neural tube occurs rostrally within the prospective brain, but previous studies have suggested that segmentation does not extend caudally into the spinal cord. We show that the xTrkB positive Rohon-Beard neurons of the spinal cord do in fact display clear segmental groupings soon after neural tube closure. This is consistent with a role for segmentation in the anterior-posterior patterning of the trunk central nervous system.