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 neurons: the trigeminal ganglion (TG) which are peripheral to the central nervous system and the mesencephalic trigeminal nucleus (MTN), the only sensory neurons 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 for the MTN neurons 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 neurons, we investigated the effect of reduced Islet-1 expression 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 direction of MTN neurons. However, outside the central nervous system a reduction of Islet-1 led to disorganized axonal growth. We observed an abnormal organisation in the maxillary division of the trigeminal nerve when Islet-1 was reduced in MTN neurons.