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

The Secretory Pathway Ca ATPases SPCA1 and SPCA2 transport Ca and Mn into the Golgi and Secretory Pathway. SPCA2 mediates store-independent Ca entry (SICE) via STIM1-independent activation of Orai1, inducing constitutive Ca influx in mammary epithelial cells during lactation. Here, we show that like SPCA2, also the overexpression of the ubiquitous SPCA1 induces cytosolic Ca influx, which is abolished by Orai1 knockdown and occurs independently of STIM1. This process elevates the Ca 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 Ca 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 Ca transport function, Orai1 activation, or both, while all mutations affect the Ca content of the non-ER stores. Thus, the functional coupling between SPCA1 and Orai1 increases cytosolic and intraluminal Ca levels, representing a novel mechanism of SICE that may be affected in HHD.