Update ArticleThe role of Cbln1 on Purkinje cell synapse formation

•Cbln1 induces excitatory synapse formation between parallel fibers and Purkinje cells.•Cbln1 forms a tripartite protein complex with neurexin and delta 2 glutamate receptors.•Parallel fibers undergo dynamic structural changes during Cbln1-induced synaptogenesis.•Cbln1 down-regulates formation and function of inhibitory synapses on Purkinje cells.

Cbln1 is a glycoprotein which belongs to the C1q family. In the cerebellum, Cbln1 is produced and secreted from granule cells and works as a strong synapse organizer between Purkinje cells and parallel fibers, the axons of the granule cells. In this update article, we will describe the molecular mechanisms by which Cbln1 induces synapse formation and will review our findings on the axonal structural changes which occur specifically during this process. We will also describe our recent finding that Cbln1 has a suppressive role in inhibitory synapse formation between Purkinje cells and molecular layer interneurons. Our results have revealed that Cbln1 plays an essential role to establish parallel fiber-Purkinje cell synapses and to regulate balance between excitatory and inhibitory input on Purkinje cells.