Important Contribution of α-Neurexins to Ca2+-Triggered Exocytosis of Secretory Granules

Abstract

α-Neurexins constitute a family of neuronal cell surface molecules that are essential for efficient neurotransmission, because mice lacking two or all three α-neurexin genes show a severe reduction of synaptic release. Although analyses of α-neurexin knock-outs and transgenic rescue animals suggested an involvement of voltage-dependent Ca²⁺channels, it remained unclear whether α-neurexins have a general role in Ca²⁺-dependent exocytosis and how they may affect Ca²⁺channels. Here we show by membrane capacitance measurements from melanotrophs in acute pituitary gland slices that release from endocrine cells is diminished by >50% in adult α-neurexin double knock-out and newborn triple knock-out mice. There is a reduction of the cell volume in mutant melanotrophs; however, no ultrastructural changes in size or intracellular distribution of the secretory granules were observed. Recordings of Ca²⁺currents from melanotrophs, transfected human embryonic kidney cells, and brainstem neurons reveal that α-neurexins do not affect the activation or inactivation properties of Ca²⁺channels directly but may be responsible for coupling them to release-ready vesicles and metabotropic receptors. Our data support a general and essential role for α-neurexins in Ca²⁺-triggered exocytosis that is similarly important for secretion from neurons and endocrine cells.