Store-independent coupling between the Secretory Pathway Ca²⁺ transport ATPase SPCA1 and Orai1 in Golgi stress and Hailey-Hailey disease

The Secretory Pathway Ca²⁺ ATPases SPCA1 and SPCA2 transport Ca²⁺ and Mn²⁺ into the Golgi and Secretory Pathway. SPCA2 mediates store-independent Ca²⁺ entry (SICE) via STIM1-independent activation of Orai1, inducing constitutive Ca²⁺ influx in mammary epithelial cells during lactation. Here, we show that like SPCA2, also the overexpression of the ubiquitous SPCA1 induces cytosolic Ca²⁺ influx, which is abolished by Orai1 knockdown and occurs independently of STIM1. This process elevates the Ca²⁺ concentration in the cytosol and in the non-endoplasmic reticulum (ER) stores, pointing to a functional coupling between Orai1 and SPCA1. In agreement with this, we demonstrate via Total Internal Reflection Fluorescence microscopy that Orai1 and SPCA1a co-localize near the plasma membrane. Interestingly, SPCA1 overexpression also induces Golgi swelling, which coincides with translocation of the transcription factor TFE3 to the nucleus, a marker of Golgi stress. The induction of Golgi stress depends on a combination of SPCA1 activity and SICE, suggesting a role for the increased Ca²⁺ level in the non-ER stores. Finally, we tested whether impaired SPCA1a/Orai1 coupling may be implicated in the skin disorder Hailey-Hailey disease (HHD), which is caused by SPCA1 loss-of-function. We identified HHD-associated SPCA1a mutations that impair either the Ca²⁺ transport function, Orai1 activation, or both, while all mutations affect the Ca²⁺ content of the non-ER stores. Thus, the functional coupling between SPCA1 and Orai1 increases cytosolic and intraluminal Ca²⁺ levels, representing a novel mechanism of SICE that may be affected in HHD.