We generated mice deficient in ADAM33 to evaluate the potential roles of this protein in development, fertility, and regulation of airway reactivity and allergic inflammation. Based on established roles for several ADAM family members in the development of a wide variety of tissues and of at least three family members in fertility, it was reasonable to hypothesize that ADAM33 might also contribute to these processes. However, we could not identify any nonredundant role for ADAM33 in either development or fertility.
Asthma is a complex disorder in which major genetic and environmental factors interact to both initiate the disease and modify its progression. The genetics of asthma are now being characterized using gene discovery and functional genetic methodologies. In 2002, Van Eerdewegh et al. reported fine mapping of the ADAM33 gene as an asthma and airway hyperreactivity gene on chromosome 20p13 by using a genome-wide scan from 460 Caucasian families (39
). Several studies have confirmed the association of ADAM33 with asthma (18
); however, Lind et al. could not find any association in Puerto Rican or Mexican populations (27
). There was no association seen in Icelandic and United Kingdom populations in another study from Blakey et al. (5
). Raby et al. did not demonstrate the association of ADAM33 single nucleotide polymorphisms with asthma or airway responsiveness, and only weak associations were observed with IgE levels and total eosinophilia (34
). Furthermore, to date, no sequence variants for ADAM33 have been described as resulting in meaningful alterations in ADAM33 expression or function. The role of ADAM33 in asthma pathogenesis thus remains controversial. It remains formally possible that the reported associations with the ADAM33 gene could be due to linkage disequilibrium with disease-causing sequence variants in other nearby genes.
Our results indicate that ADAM33 is not required for normal function of airway smooth muscle in mice and does not regulate baseline airway reactivity. We were also unable to find any evidence supporting a role for ADAM33 in allergic sensitization or in the increased airway reactivity, mucus metaplasia, or airway inflammation induced by allergen challenge. Multiple methods have been used to measure airway responsiveness in mice. The method we used, based on measurements of airflow and pressure during forced oscillation maneuvers, is primarily a reflection of the caliber of the conducting airways. Unrestrained plethysmography, a noninvasive technique that requires little operator effort, has also been widely used. However, the interpretation of the changes in the pattern of breathing measured by this approach remains controversial (2
). It has also been reported that administration of bronchoconstrictor drugs by inhalation may yield results different from those obtained by administration intravenously, as we did here (12
). More importantly, murine models of asthma do not perfectly reproduce human asthma. Murine models do not perfectly mimic the role that mast cells appear to play in the human disease, nor do they perfectly mimic the airway wall remodeling that accompanies human asthma. Because mice have only a single submucosal gland in their airways, the glandular hyperplasia that is often a feature of human asthma cannot be effectively studied in the mouse. It therefore remains possible that alterations in expression or function of ADAM33 could play a role in human asthma.
Inbred mice show strain-specific variation with respect to various traits of asthma, such as airway inflammation and bronchial hyperresponsiveness (9
). We did not find differences in any asthma-associated traits in 129/SVJae mice but cannot exclude the possibility that there would be effects of ADAM33 in other genetic backgrounds. Ackerman et al. found that the difference between two mouse strains (A/J and C57BL/6J) in airway hyperresponsiveness was not associated with any single locus but was significantly associated with an interaction of loci on chromosomes 2 and 6 (1
). The mouse ADAM33 gene is located on chromosome 2. However, there was no difference between nearly 200 A/J and C57BL/6J single nucleotide polymorphisms in the ADAM33 genomic sequence (http://mousesnp.roche.com/
). Together with our finding, these results suggest that the ADAM33 gene may not contribute to phenotypic features of asthma in mice.
Other ADAM proteins play important roles in protein ectodomain shedding. Many cell surface molecules, such as cytokines, cytokine receptors, cell adhesion molecules, and growth factors, are processed to convert them into physiologically active soluble derivatives. Proteases responsible for releasing cell surface molecules are classified as sheddases or secretases. ADAM33 belongs to a subfamily that includes ADAM19, ADAM12, and ADAM13, as determined by sequence homology, (16
). ADAM19 may participate in the shedding of β1-neuregulin (37
). ADAM12 was reported to process HB-EGF and insulin-like growth factor binding proteins 3 and 5 in vivo (3
). ADAM33 was also found to be able to cleave α2-macroglobulin (15
). However, it is still not clear what the physiological substrates of ADAM33 are and if ectodomain shedding by ADAM33 contributes in any meaningful way to human biology. Similarly, like most other ADAM proteins, ADAM33 contains an integrin-binding disintegrin domain that can serve as a ligand for integrin α9β1 (10
). Integrin α9β1 binds to several other ligands, and mice lacking the α9 subunit (19
) die within the first 12 days of life. It is thus clear that neither integrin depends principally on interaction with ADAM33 for its major in vivo functions. Nonetheless, ADAM33 knockout mice and the cell lines derived from these mice should be useful tools to test and verify potential roles for this protein in other disease models and to identify and validate its proteolytic substrates and integrin interactions.