Chemoattraction and chemorepulsion of olfactory bulb axons by different secreted semaphorins

During development, growth cones can be guided at a distance by diffusible factors, which are attractants and/or repellents. The semaphorins are the l argest family of repulsive axon guidance molecules. Secreted semaphorins bi nd neuropilin receptors and repel sensory, sympathetic, motor, and forebrai n axons. We found that in rat embryos, the olfactory epithelium releases a diffusible factor that repels olfactory bulb axons. In addition, Sema A and Sema IV, but not Sema ill, Sema E, or Sema H, are able to orient in vitro the growth of olfactory bulb axons; Sema IV has a strong repulsive action, whereas Sema A appears to attract those axons, The expression patterns of s ema A and sema IV in the developing olfactory system confirm that they may play a cooperative role in the formation of the lateral olfactory tract. Th is also represents a further evidence for a chemoattractive function of sec reted semaphorins.