ISLET-1 knockdown causes abnormal peripheral axonal growth of mesencephalic trigeminal neurones in the chick embryo

The trigeminal is one of the best characterized sensory systems in amniotes. It comprises two populations of first-order sensory neurones: the trigeminal ganglion (TG) which are peripheral to the central nervous system and the mesencephalic trigeminal nucleus (MTN), the only sensory neurones that lie within the central nervous system in amniotes. Islet-1, a LIM homeodomain transcription factor which plays essential roles during embryogenesis, contributes to axon pathfinding of sensory neurons in the TG. However, if Islet-1 plays a similar role in the MTN neurones is unknown. To answer whether Islet-1 is as important for axon guidance in these centrally-located sensory neurons as it is for the peripherally-located TG neurones, we investigated the effect of disrupting Islet-1 on axonal pathfinding in the chick MTN. We employed in ovo electroporation to transfect short interfering RNA for Islet-1 (si-Islet-1) into the dorsal midbrain. Our findings showed that, within the central nervous system, Islet-1 knockdown did not affect axonal growth of MTN neurones. However, reduction of Islet-1 in dorsal midbrain cells led to disorganized axonal growth once outside the central nervous system. As a consequence, we observed an abnormal organisation in the maxillary division of the trigeminal nerve in these embryos.