Impact of Calreticulin and Its Mutants on Endoplasmic Reticulum Function in Health and Disease

Membranal Expression of Calreticulin Induced by Unfolded Protein Response in Melanocytes: A Mechanism Underlying Oxidative Stress-Induced Autoimmunity in Vitiligo

Calreticulin (CRT), a damage-associated molecular pattern molecule, is reported to translocate from the endoplasmic reticulum to the membrane in melanocytes under oxidative stress. To investigate the potential role of CRT in the pathogenesis of vitiligo, we analyzed the correlation between CRT and ROS in serum and lesions of vitiligo, detected CRT and protein kinase RNA-like endoplasmic reticulum kinase (PERK) expression in vitiligo lesions, and studied the production of CRT and mediators of unfolded protein response (UPR) pathway and then tested the chemotactic migration of CD8+ T cells or CD11c+ CD86+ cells. Initially, we verified the overexpression of CRT in perilesional epidermis that was positively correlated with the disease severity of vitiligo. Furthermore, the PERK branch of UPR was confirmed to be responsible for the overexpression and membranal translocation of CRT in melanocytes under oxidative stress. We also found that oxidative stress-induced membranal translocation of CRT promoted the activation and migration of CD8+ T cells in vitiligo. In addition, dendritic cells from patients with vitiligo were also prone to maturation with the coincubation of melanocytes harboring membranal CRT. CRT could be induced on the membrane of melanocytes through UPR and might play a role in oxidative stress-triggered CD8+ T-cell response in vitiligo.

Calreticulin: Endoplasmic reticulum Ca2+ gatekeeper

Endoplasmic reticulum (ER) luminal Ca2+ is vital for the function of the ER and regulates many cellular processes. Calreticulin is a highly conserved, ER-resident Ca2+ binding protein and lectin-like chaperone. Over four decades of studying calreticulin demonstrate that this protein plays a crucial role in maintaining Ca2+ supply under different physiological conditions, in managing access to Ca2+ and how Ca2+ is used depending on the environmental events and in making sure that Ca2+ is not misused. Calreticulin plays a role of ER luminal Ca2+ sensor to manage Ca2+ -dependent ER luminal events including maintaining interaction with its partners, Ca2+ handling molecules, substrates and stress sensors. The protein is strategically positioned in the lumen of the ER from where the protein manages access to and distribution of Ca2+ for many cellular Ca2+ -signalling events. The importance of calreticulin Ca2+ pool extends beyond the ER and includes influence of cellular processes involved in many aspects of cellular pathophysiology. Abnormal handling of the ER Ca2+ contributes to many pathologies from heart failure to neurodegeneration and metabolic diseases.