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A ‘don’t eat me’ immune signal protects neuronal connections
During development, some synaptic connections between neurons are removed by immune cells called microglia, and others are retained. The discovery of a ‘don’t eat me’ signal that prevents excess pruning sheds light on this process.
The signals transmitted between neurons through synaptic connections are responsible for most, if not all, brain functions, from learning to decision-making. During brain development, synapses that are stimulated less often than others are eliminated through a process called pruning, whereas those that are highly stimulated are retained. This refines the brain’s ability to respond to stimuli and environmental cues. Microglia, the brain’s innate immune cells, have a key role in pruning — they engulf and digest synapses through a process called phagocytosis. But the mechanism that determines which synapses they avoid has been unclear. Writing in Neuron, Lehrman et al.1 describe a ‘don’t eat me’ signal, involving a protein called cluster of differentiation 47 (CD47), that prevents inappropriate synaptic pruning by microglia.