ER-to-Golgi blockade of nascent desmosomal cadherins in SERCA2-inhibited keratinocytes: Implications for Darier's disease.

Darier's disease (DD) is an autosomal dominantly inherited skin disorder caused by mutations in sarco/endoplasmic reticulum Ca2+-ATPase 2 (SERCA2), a Ca2+ pump that transports Ca2+ from the cytosol to the endoplasmic reticulum (ER). Loss of desmosomes and keratinocyte cohesion is a characteristic feature of DD. Desmosomal cadherins (DC) are Ca2+-dependent transmembrane adhesion proteins of desmosomes, which are mislocalized in the lesional but not perilesional skin of DD. We show here that inhibition of SERCA2 by 2 distinct inhibitors results in accumulation of DC precursors in keratinocytes, indicating ER-to-Golgi transport of nascent DC is blocked. Partial loss of SERCA2 by siRNA has no such effect, implicating that haploinsufficiency is not sufficient to affect nascent DC maturation. However, a synergistic effect is revealed between SERCA2 siRNA and an ineffective dose of SERCA2 inhibitor, and between an agonist of the ER Ca2+ release channel and SERCA2 inhibitor. These results suggest that reduction of ER Ca2+ below a critical level causes ER retention of nascent DC. Moreover, colocalization of DC with ER calnexin is detected in SERCA2-inhibited keratinocytes and DD epidermis. Collectively, our data demonstrate that loss of SERCA2 impairs ER-to-Golgi transport of nascent DC, which may contribute to DD pathogenesis.