Stepwise molecular specification of excitatory synapse diversity on a target neuron

Brain function relies on the generation of a large variety of morphologically and functionally diverse, but specific, neuronal synapses. Here, we show that, initially, synapse formation on a common target neuron, the cerebellar Purkinje cells, involves a presynaptic secreted protein common for all types of excitatory inputs. The molecular program then evolves only in one of the inputs with the additional expression of a combination of presynaptic secreted proteins that specify the mature pattern of connectivity on the target. These results show that some inputs actively and gradually specify their synaptic molecular identity while others rely on the "original code". Thus, the molecular specification of excitatory synapses, crucial for proper circuit function, is acquired in a stepwise manner during mouse postnatal development and obeys input-specific rules.