Myocilin is secreted from trabecular meshwork cells to an eponymous extracellular matrix that is critical for maintaining intraocular pressure. Missense mutations found in the
myocilin olfactomedin domain (OLF) lead to intracellular
myocilin misfolding and are causative for the heritable form of
early-onset glaucoma. The OLF domain contains a unique internal, hetero-dinuclear
calcium site. Here, we tested the hypothesis that
calcium dysregulation causes wild-type (WT)
myocilin misfolding reminiscent of that observed for disease variants. Using two cellular models expressing WT
myocilin, we show that the
Ca2+ ATPase channel blocker
thapsigargin inhibits WT
myocilin secretion. Intracellular WT
myocilin is at least partly insoluble and aggregated in the endoplasmic reticulum (ER), and stains positively with an
amyloid dye. By comparing the effect of thapsigargin on WT
myocilin to that on a de novo secretion-competent Ca2+-free variant D478S, we discern that non-secretion of WT
myocilin is due initially to
calcium dysregulation, and is potentiated further by resultant ER stress. In E. coli, depletion of
calcium leads to recombinant expression of misfolded isolated WT OLF but the D478S variant is still produced as a folded monomer. Treatment of cells expressing a double mutant composed of D478S and either disease variants P370L or Y437H with
thapsigargin promotes its misfolding and aggregation, demonstrating the limits of D478S to correct secretion defects. Taken together, the heterodinuclear
calcium site is a liability for proper folding of
myocilin. Our study suggests a molecular mechanism by which WT
myocilin misfolding may contribute broadly to
glaucoma-associated ER stress. This study explores the effect of
calcium depletion on
myocilin olfactomedin domain folding.