SPLUNC1 α6 Peptidomimetic: A Novel Therapeutic for Asthma.

Asthma is a disease characterized by chronic airway inflammation and reversible airflow limitation. This inflammation involves activated mast cells, infiltration of eosinophils, neutrophilia, mucus accumulation, and increases in the number of activated T-helper cell type 2 cells, which orchestrate and perpetuate allergic inflammation through the release of various cytokines and chemokines (1, 2). People with severe asthma experience persistent symptoms despite the use of conventional medications including long-acting β2-adrenergic receptor agonists, muscarinic antagonists, leukotriene receptor antagonists, and inhaled corticosteroids for reasons that are not fully understood (1, 2). Therefore, there is a clinical and urgent need for new therapies to effectively treat people with severe asthma.

It is well recognized that many of the pathogenic processes in asthmatic airways are dependent on increases in cytoplasmic Ca2+ and include events such as excitation–contraction coupling in smooth muscle, stimulus–secretion coupling in mast cells and mucous glands, nerve impulse initiation and conduction, and inflammatory cell infiltration (2, 3). Orai1 is a plasma membrane Ca2+ channel that is involved in store-operated Ca2+ entry, and Orai1/store-operated Ca2+ entry are upstream of transcription factors that promote inflammation (4–6). It is therefore conceivable that inhibition of Orai1 could be a novel way of reducing inflammation in a range of conditions including asthma. Wu and colleagues have previously shown that the multifunctional protein SPLUNC1 (Short Palate LUng and Nasal epithelial Clone 1), via its sixth α-helical (α6) region, regulates Orai1 and Ca2+ signaling (7). Moreover, studies have shown that SPLUNC1 is expressed in airway epithelia and secreted into the lung lumen where it regulates processes such as smooth muscle contraction and inflammation (7–10). Thus, there is a case for the development of peptidomimetics of SPLUNC1’s α6 region to inhibit Orai1 to reduce pulmonary inflammation.

In this issue of the Journal, Wrennall and colleagues (pp. 271–282) report on their examination of the therapeutic potential of a novel α6 peptidomimetic for asthma (11). They did this by first investigating the role of SPLUNC1 in allergy-induced asthma by exposing wild-type (WT) and SPLUNC1−/− mice to house dust mite (HDM) allergen. It was interesting to note that HDM exposure resulted in a significantly greater increase in eosinophil counts in SPLUNC1−/− mice than in WT littermates, suggesting a hyperinflammatory response. These findings are consistent with those of Thaikoottathil and colleagues showing that SPLUNC1 deficiency enhances airway eosinophilic inflammation in mice (9), indicating that SPLUNC1 may play a role in modulating the T-helper cell type 2 immune response in the lung. The concept of SPLUNC1 playing a role in asthma is further supported by studies showing that SPLUNC1 is a gene modifier for asthma (12) and that SPLUNC1 levels are reduced in people with asthma (7).

Having established a role for SPLUNC1 in allergic inflammation, the authors then proceeded to generate a peptide based on SPLUNC1’s α6 region to test whether it could reduce Orai1 levels in HDM-exposed mice after intranasal delivery. BAL immune cells from SPLUNC1−/− mice treated with HDM had significantly greater Orai1 expression than BAL cells obtained from WT littermate control mice. Interestingly, intranasally delivered α6 reduced Orai1 expression in BAL immune cells obtained from both WT and SPLUNC1−/− mice, suggesting that Orai1 expression in BAL immune cells may play a role in the sensitization of inflammatory responses to allergens and that this effect may be regulated by SPLUNC1. Given that SPLUNC1−/− mice displayed exaggerated inflammatory responses to HDM exposure and that α6 reduced Orai1 expression in BAL immune cells, it was no surprise that the α6 peptide inhibited airway inflammation in HDM-exposed mice.

The therapeutic potential of aerosolized therapies for asthma is dependent on several factors including the particle size of the drug, the dose deposited, its distribution within the lung, and whether pathological features of the disease impede drug delivery via inhalation. Given that it is well recognized that mucus can interact with proteins and peptides, the authors accordingly sought to determine whether their formulated α6 peptide interacts with mucus, which would impede delivery via inhalation. Quartz crystal microbalance with dissipation analysis demonstrated that α6 does not interact with mucus. Moreover, scanning EM revealed that jet milled α6 displayed a consistent particle size that was suitable for inhalation to the lower airways. Collectively, the data indicate that the α6 peptide is a viable candidate for dry powder inhalation and importantly that its activity was not affected by jet milling or nebulization.

There are potential limitations to this study that bear some discussion. First, the authors have shown that α6 neither interacts with nor penetrates the epithelial barrier, suggesting that α6 imparts its antiinflammatory effects solely through interactions with immune cells already recruited to the airway lumen. The authors propose that this could potentially be through α6 reducing Ca2+-dependent cytokine release from these cells and/or reducing the residence time of eosinophils in the airway lumen. Further mechanistic studies to fully understand the extent of α6 penetration into the lung, and its target cell types, are required. Second, it is well recognized that people with severe asthma have significant airway wall remodeling. Although the authors show antiinflammatory effects of the α6 peptidomimetic, it would have been worth investigating whether it can also reduce airway wall remodeling observed in severe asthma. In saying this, the authors have shown in other studies that the α6-related peptides reduce histological signs of airway damage in murine models of bacterial pneumonia, suggesting that α6 may provide protection against airway remodeling. Finally, the authors show that the α6 peptidomimetic reduces lung inflammation, but it would have been worth investigating whether this reduced lung inflammation translated to improvements in lung function. Despite these potential limitations, this valuable study will inform and prompt further studies in the field of asthma that will expand on the outcomes of the present study.

In summary, this study by Wrennall and colleagues has shown that administration of a SPLUNC1 α6 peptidomimetic reduces HDM-induced airway inflammation in mice and that the α6 peptide may serve as a novel therapeutic for the treatment of asthma where conventional treatments do not work or where nonsteroidal alternatives are needed.