Essential role of Orai1 store-operated calcium channels in lactation

The nourishment of neonates by nursing is the defining characteristic of mammals. However, despite considerable research into the neural control of lactation, an understanding of the signaling mechanisms underlying the production and expulsion of milk by mammary epithelial cells during lactation remains largely unknown. Here we demonstrate that a store-operated Ca ²⁺ channel subunit, Orai1, is required for both optimal Ca ²⁺ transport into milk and for milk ejection. Using a novel, 3D imaging strategy, we visualized live oxytocin-induced alveolar unit contractions in the mammary gland, and we demonstrated that in this model milk is ejected by way of pulsatile contractions of these alveolar units. In mammary glands of Orai1 knockout mice, these contractions are infrequent and poorly coordinated. We reveal that oxytocin also induces a large transient release of stored Ca ²⁺ in mammary myoepithelial cells followed by slow, irregular Ca ²⁺ oscillations. These oscillations, and not the initial Ca ²⁺ transient, are mediated exclusively by Orai1 and are absolutely required for milk ejection and pup survival, an observation that redefines the signaling processes responsible for milk ejection. These findings clearly demonstrate that Ca ²⁺ is not just a substrate for nutritional enrichment in mammals but is also a master regulator of the spatiotemporal signaling events underpinning mammary alveolar unit contraction. Orai1-dependent Ca ²⁺ oscillations may represent a conserved language in myoepithelial cells of other secretory epithelia, such as sweat glands, potentially shedding light on other Orai1 channelopathies, including anhidrosis (an inability to sweat). Significance All mammals, from platypuses to humans, produce relatively immature offspring that are wholly dependent on their mother’s milk for their postnatal growth and development. However, the dynamic signaling and molecular mechanisms responsible for the transport of key constituents (e.g., calcium) into milk and for alveolar unit contraction and milk ejection are not fully understood. Using genetically modified mouse models, we demonstrate that the store-operated Ca ²⁺ channel Orai1 delivers over 50% of the calcium ions present in milk. We also reveal an unanticipated role of Orai1 as a master regulator of oxytocin-mediated alveolar unit contractility, milk ejection, and pup survival. These results provide a unique mechanistic insight into the fundamentally mammalian process of lactation.