Alterations of protein folding or Ca²⁺ levels within the endoplasmic reticulum (ER) result in the unfolded-protein response (UPR), a process considered as an endogenous inducer of inflammation. Thereby, understanding how genetic factors modify UPR is particularly relevant in chronic inflammatory diseases such as asthma. Here we identified that ORMDL3, the only genetic risk factor recently associated to asthma in a genome wide study, alters ER-mediated Ca²⁺ homeostasis and facilitates the UPR. Heterologous expression of human ER-resident transmembrane ORMDL3 protein increased resting cytosolic Ca²⁺ levels and reduced ER-mediated Ca²⁺ signaling, an effect reverted by co-expression with the sarco-endoplasmic reticulum Ca²⁺ pump (SERCA). Increased ORMDL3 expression also promoted stronger activation of UPR transducing molecules and target genes while siRNA-mediated knock-down of endogenous ORMDL3 potentiated ER Ca²⁺ release and attenuated the UPR. In conclusion, our findings are consistent with a model in which ORMDL3 binds and inhibits SERCA resulting in a reduced ER Ca²⁺ concentration and increased UPR. Thus, we provide a first insight into the molecular mechanism explaining the association of ORMDL3 with proinflammatory diseases.

Transient receptor potential cation channels of the vanilloid subfamily (TRPV) participate in the generation of Ca(2+) signals at different locations of the respiratory system, thereby controlling its correct functioning. TRPV1 expression and activity appear to be altered under pathophysiological conditions such as chronic cough and airway hypersensitivity, whereas TRPV4 single nucleotide polymorphisms (SNP) are associated with chronic obstructive pulmonary disease. However, to date, there is no information about the genetic impact of either TRPV1 or TRPV4 on asthma pathophysiology. We now report on the association of two functional SNPs, TRPV1-I585V and TRPV4-P19S, with childhood asthma. Both SNPs were genotyped in a population of 470 controls without respiratory symptoms and 301 asthmatics. Although none of the SNPs modified the risk of suffering from asthma, carriers of the TRPV1-I585V genetic variant showed a lower risk of current wheezing (odds ratio = 0.51; p = 0.01), a characteristic of active asthma, or cough (odds ratio = 0.57; p = 0.02). Functional analysis of TRPV1-I585V, using the Ca(2+)-sensitive dye fura-2 to measure intracellular [Ca(2+)] concentrations, revealed a decreased channel activity in response to two typical TRPV1 stimuli, heat and capsaicin. On the other hand, TRPV4-P19S, despite its loss-of-channel function, showed no significant association with asthma or the presence of wheezing. Our data suggest that genetically determined level of TRPV1 activity is relevant for asthma pathophysiology.

T lymphocytes rely on a Ca²⁺ signal known as store-operated calcium entry (SOCE) for their activation. This Ca²⁺ signal is generated by activation of a T-cell receptor, depletion of endoplasmic reticulum (ER) Ca²⁺ stores and activation of Ca²⁺ release-activated Ca²⁺ currents (I_CRAC). Here, we report that the ER protein orosomucoid like 3 (ORMDL3), the product of the ORMDL3 gene associated with several autoimmune and/or inflammatory diseases, negatively modulates I_CRAC, SOCE, nuclear factor of activated T cells nuclear translocation and interleukin-2 production. ORMDL3 inhibits the Ca²⁺ influx mechanism at the outer mitochondrial membrane, resulting in a Ca²⁺-dependent inhibition of I_CRAC and reduced SOCE. The effect of ORMDL3 could be mimicked by interventions that decreased mitochondrial Ca²⁺ influx and reverted by buffering of cytosolic Ca²⁺ or activation of mitochondrial Ca²⁺ influx. In conclusion, ORMDL3 modifies key steps in the process of T-lymphocyte activation, providing a functional link between the genetic associations of the ORMDL3 gene with autoimmune and/or inflammatory diseases.

Mucin 5AC (MUC5AC) is secreted by goblet cells of the respiratory tract and, surprisingly, also expressed de novo in mucus secreting cancer lines. siRNA-mediated knockdown of 7343 human gene products in a human colonic cancer goblet cell line (HT29-18N2) revealed new proteins, including a Ca²⁺-activated channel TRPM5, for MUC5AC secretion. TRPM5 was required for PMA and ATP-induced secretion of MUC5AC from the post-Golgi secretory granules. Stable knockdown of TRPM5 reduced a TRPM5-like current and ATP-mediated Ca²⁺ signal. ATP-induced MUC5AC secretion depended strongly on Ca²⁺ influx, which was markedly reduced in TRPM5 knockdown cells. The difference in ATP-induced Ca²⁺ entry between control and TRPM5 knockdown cells was abrogated in the absence of extracellular Ca²⁺ and by inhibition of the Na⁺/Ca²⁺ exchanger (NCX). Accordingly, MUC5AC secretion was reduced by inhibition of NCX. Thus TRPM5 activation by ATP couples TRPM5-mediated Na⁺ entry to promote Ca²⁺ uptake via an NCX to trigger MUC5AC secretion.

Mucins are the main macrocomponents of the mucus layer that protects the digestive tract from pathogens. Fucosylation of mucins increases mucus viscoelasticity and its resistance to shear stress. These properties are altered in patients with ulcerative colitis (UC), which is marked by a chronic inflammation of the distal part of the colon. Here, we show that levels of Fucosyltransferase 8 (FUT8) and specific mucins are increased in the distal inflamed colon of UC patients. Recapitulating this FUT8 overexpression in mucin-producing HT29-18N2 colonic cell line increases delivery of MUC1 to the plasma membrane and extracellular release of MUC2 and MUC5AC. Mucins secreted by FUT8 overexpressing cells are more resistant to removal from the cell surface than mucins secreted by FUT8-depleted cells (FUT8 KD). FUT8 KD causes intracellular accumulation of MUC1 and alters the ratio of secreted MUC2 to MUC5AC. These data fit well with the Fut8 ^−/− mice phenotype, which are protected from UC. Fut8 ^−/− mice exhibit a thinner proximal colon mucus layer with an altered ratio of neutral to acidic mucins. Together, our data reveal that FUT8 modifies the biophysical properties of mucus by controlling levels of cell surface MUC1 and quantity and quality of secreted MUC2 and MUC5AC. We suggest that these changes in mucus viscoelasticity likely facilitate bacterial–epithelial interactions leading to inflammation and UC progression.