The interconnected relationships between middle ear bulla size, cavitation defects, and chronic otitis media revealed in a syndromic mouse model

The A2ml1-Knockout mouse as an animal model for non-syndromic otitis media

BACKGROUND

Inflammation and infection of the middle ear, known as otitis media (OM), is a leading cause of hearing loss and the most frequently diagnosed disease in children worldwide. Traditionally, mouse models for OM rely on inducing acute infection through inoculation of the middle ear, e.g. with the human otopathogen non-typeable Haemophilus influenzae (NTHi), and with very few genetic models with spontaneous or chronic OM. A2ML1 variants, including loss-of-function variants, were associated with susceptibility to OM in humans, but no animal model has been reported for A2ml1-related OM. Here, we report our middle ear findings in a mouse line with a CRISPR-induced knockout (KO) of A2ml1.

METHODS

Mice were X-rayed prior to harvest to determine if there are craniofacial or skeletal abnormalities. Tissue from mouse middle ears, as well as other upper respiratory mucosal tissues, were harvested. The harvested middle ear bullae were examined under microscope and submitted for histologic preparation to study phenotypic indications of OM. RNA samples isolated from middle ear tissue were assayed for expression of genes correlated with A2ML1 expression in humans.

RESULTS

Data from a total of 119 mice (35 wildtype, 40 heterozygous, 44 homozygous) are presented here, with each analyses being performed on subsets of these mice. There were no significant craniofacial differences by genotype (n = 22). Findings in mice with the A2ml1-KO indicated an increased incidence of OM (n=29; odds ratio = 11; CI: 1.1, 573.6; Fisher exact two-sided p = 0.02) with tympanic membrane perforations or thickening, as well as cases of middle ear effusion, inflammatory cells, or fluid from histologic sections. Dsp was upregulated in the middle ear tissues of homozygous mice (Wilcoxon test p = 0.001).

CONCLUSION

Thus far, our results in this A2ml1-KO mouse line indicate spontaneous occurrence of OM and dysregulation of Dsp in the middle ear as a potential disease mechanism for A2ml1-related OM.

Tracking cell layer contribution during repair of the tympanic membrane

The tympanic membrane (i.e. eardrum) sits at the interface between the middle and external ear. The tympanic membrane is composed of three layers: an outer ectoderm-derived layer, a middle neural crest-derived fibroblast layer with contribution from the mesoderm-derived vasculature, and an inner endoderm-derived mucosal layer. These layers form a thin sandwich that is often perforated following trauma, pressure changes or middle ear inflammation. During healing, cells need to bridge the perforation in the absence of an initial scaffold. Here, we assessed the contribution, timing and interaction of the different layers during membrane repair by using markers and reporter mice. We showed that the ectodermal layer is retracted after perforation, before proliferating away from the wound edge, with keratin 5 basal cells migrating over the hole to bridge the gap. The mesenchymal and mucosal layers then used this scaffold to complete the repair, followed by advancement of the vasculature. Finally, differentiation of the epithelium led to formation of a scab. Our results reveal the dynamics and interconnections between the embryonic germ layers during repair and highlight how defects might occur.

PANEL 3: Otitis media animal models, cell culture, tissue regeneration & pathophysiology

OBJECTIVE

To review and summarize recently published key articles on the topics of animal models, cell culture studies, tissue biomedical engineering and regeneration, and new models in relation to otitis media (OM).

DATA SOURCE

Electronic databases: PubMed, National Library of Medicine, Ovid Medline.

REVIEW METHODS

Key topics were assigned to the panel participants for identification and detailed evaluation. The PubMed reviews were focused on the period from June 2019 to June 2023, in any of the objective subject(s) or keywords listed above, noting the relevant references relating to these advances with a global overview and noting areas of recommendation(s). The final manuscript was prepared with input from all panel members.

CONCLUSIONS

In conclusion, ex vivo and in vivo OM research models have seen great advancements in the past 4 years. From the usage of novel genetic and molecular tools to the refinement of in vivo inducible and spontaneous mouse models, to the introduction of a wide array of reliable middle ear epithelium (MEE) cell culture systems, the next five years are likely to experience exponential growth in OM pathophysiology discoveries. Moreover, advances in these systems will predictably facilitate rapid means for novel molecular therapeutic studies.

Otitis media: Interactions between host and environment, immune and inflammatory responses

OBJECTIVE

To review and highlight progress in otitis media (OM) research in the areas of immunology, inflammation, environmental influences and host-pathogen responses from 2019 to 2023. Opportunities for innovative future research were also identified.

DATA SOURCES

PubMed database of the National Library of Medicine.

REVIEW METHODS

Key topics were assigned to each panel member for detailed review. Search of the literature was from June 2019 until February 2023. Draft reviews were collated, circulated, and discussed among panel members at the 22nd International Symposium on Recent Advances in Otitis Media in June 2023. The final manuscript was prepared and approved by all the panel members.

CONCLUSIONS

Important advances were identified in: environmental influences that enhance OM susceptibility; polymicrobial middle ear (ME) infections; the role of adaptive immunity defects in otitis-proneness; additional genes linked to OM; leukocyte contributions to OM pathogenesis and recovery; and novel interventions in OM based on host responses to infection. Innovative areas of research included: identification of novel bacterial genes and pathways important for OM persistence, bacterial adaptations and evolution that enhance chronicity; animal and human ME gene expression, including at the single-cell level; and Sars-CoV-2 infection of the ME and Eustachian tube.

Role of a novel mouse mutant of the Galnt2 tm1Lat/tm1Lat gene in otitis media

Genetic susceptibility is one of the most important causes of otitis media (OM). Mutant Galnt2 homozygote (Galnt2 tm1Lat/tm1Lat) mimics human otitis media in comparable pathology and causes hearing loss. Otitis media is characterized by effusion and dysregulated mucosa proliferation and capillary expansion in the middle ear cavity, which is associated with hearing loss. The mucociliary dysfunction could be seen in the middle ear cavity (MEC) in a patient harboring the disease that develops in severity with age by a scanning electron microscope. Tumor necrosis factor alpha (TNF-α), transforming growth factor-beta 1 (TGF-β1), Muc5ac, and Muc5b upregulate the expression in the middle ear, which correlates with inflammation, craniofacial development, and mucin secretion. The mouse model with a mutation in the Galnt2 (Galnt2 tm1Lat/tm1Lat) was explored in this study as a novel model of human otitis media.