The many faces of Orai1 dysfunction: Molecular mechanisms and clinical manifestations

Orai1, the pore-forming subunit of the calcium (Ca2+) release-activated Ca2+ (CRAC) channel, plays a central role in store-operated Ca2+ entry (SOCE) in animal cells and thereby serves as a key regulator of intracellular Ca2+ homeostasis. Disruption of this tightly controlled process is associated with a wide spectrum of human diseases. Dysfunction can result from a multitude of remodeling mechanisms, including altered protein expression (up- or downregulation), assembly remodeling, or mutations. We focus in particular on Orai1 mutations, which have been linked to severe combined immunodeficiency (SCID) as a result of channel loss-of-function (LoF), as well as to disorders like tubular aggregate myopathy (TAM) and Stormorken syndrome (STRMK) arising from gain-of-function (GoF) alterations. These mutation-induced functional defects can be attributed to a wide variety of disruptions in the complex activation cascade of the Orai1 channel. Under physiological conditions, Orai1 activation involves all four transmembrane (TM) domains and follows a sophisticated interaction mechanism that ensures accurate signal transmission from the protein periphery toward its central Ca2+-conducting pore. In this Review, we compile all currently known disease-associated Orai1 mutations, delineate the mechanisms by which they interfere with the activation cascade, and discuss their pathological relevance. Their widespread distribution across all the domains of this Ca2+ channel highlights that malfunctions at virtually any point along the Orai1 TM domain interfaces can profoundly impair its activation mechanism, ultimately leading to severe diseases.