As one of the most common respiratory diseases, asthma affects over 300 million people worldwide. It is estimated that approximately 100 million new cases of asthma will be diagnosed by 2025 (Dharmage et al., 2019). However, asthma is not curable; the contemporary management paradigm is mainly focused on symptom control and the avoidance of triggers (Yaghoubi et al., 2019). The gene-environment interaction plays a predominant role in the pathogenesis of asthma (Yaghoubi et al., 2019). Thus, it is crucial to investigate the molecular mechanism underlying asthma attacks in response to environmental stimuli.

Epidemiological studies have suggested that cold exposure is associated with an increased risk of asthma attacks in both children and adults (Xu et al., 2018, Lam et al., 2016, Kabir et al., 2021, Ueda et al., 2010, Cong et al., 2017, Shoraka et al., 2019). A meta-analysis of 26 studies, with over 26 million participants across 14 countries, showed that a 1 °C drop in temperature was associated with a 5% increased risk of asthma attacks (Cong et al., 2017). Several animal models have been established to explore cold-induced asthma exacerbations. Liao et al. suggested that airway inflammation and hyper-responsiveness can be prominently relieved by housing asthmatic mice at a thermoneutral temperature of 30 °C, compared with a standard temperature of 20 °C (Liao et al., 2017). Moreover, Deng et al. and Du et al. showed that repeated cold exposure at 10 °C can aggravate asthmatic symptoms in mice (Deng et al., 2020, Du et al., 2019). However, the exact mechanisms underlying the development of asthma under cold conditions remain unclear.

Humans can maintain a body core temperature of 37 °C despite fluctuations in the ambient temperature. However, organs directly exposing to the external environment, such as the skin and trachea, can be significantly affected by external temperature changes. McFadden et al. reported that human airway temperature may drop below 30 ℃ during cold exposure and physical activities; thus, airway temperature fluctuations may be related to cold-related asthmatic exacerbations (McFadden et al., 1985).

The bronchial epithelium constitutes an interface between internal tissue compartment and external environment, and it plays a pivotal role in maintaining tissue homeostasis by providing a physical barrier with tight junction proteins, such as occludin, claudin, and zona occludens-1 (ZO-1) (Xiao et al., 2011). Bronchial tight junctions are compromised in patients with asthma, which facilitates epithelial barrier penetration by pathogens or other allergens, thereby triggering an immune response (Lee et al., 2018, Hellings and Steelant, 2020). It has been suggested that proinflammatory cytokines such as IL-4 and TNF-a, as well as histamine can cause decreased occludin and ZO-1 expression, which led to disrupted epithelial integrity (Kubo et al., 2015, Steelant et al., 2016, Steelant et al., 2018). However, it remains unknown whether low temperature may impair the epithelial barrier by reducing the expression of tight junction proteins. Therefore, in this study, we aimed to determine the effects of cold exposure on occludin expression, and its implication in cold-induced asthma attacks. We hypothesized that low temperature exposure may reduce occludin expression, and cause epithelial barrier defects, which in turn, results in asthma exacerbation. Furthermore, Nedd4–2 (neural precursor cell expressed developmentally down-regulated 4-like) is a ubiquitin protein ligase which was found to bind and regulate occludin expression to aid in its turnover. The catalytic activity of Nedd4–2 can be regulated either by SGK1 (glucocorticoid-inducible kinase)-mediated phosphorylation or calcium binding (Pohl et al., 2021). Thus, we also investigated the potential mechanisms underlying cold-induced occludin alteration by examining the SGK1/Nedd4–2 pathways and Ca2+ channels.