As we grow from infancy to adulthood, specific connections (synapses) between nerve cells are formed, and excess connections pruned away. Approximately 40% of the connections (synapses) between nerve cells in our brains disappear during development. How does a nerve cell decide which synapses to destroy? Din et al., studying the nematode Caenorhabditis elegans provide evidence that the creation of adult synapses triggers the destruction of developmentally transient synapses forged by the same neuron. David Miller offers a summary figure in his review showing the molecular details of how a primary synapse region matures while a secondary synapse region is eliminated.
Disconnections (Click to enlarge). (Top) The developing HSNL motor neuron initially forms synapses with vulval muscles and motor neurons in two locations. (Bottom) The protein SYG-1 blocks proteolysis of synaptic proteins at primary synapses but allows destruction of secondary synapses. E2, E2 ubiquitin conjugating enzyme; RBX, Ring finger protein.