Unravelling the role of cochlin in the innate immune response of the cochlea

Ondertitel:impact of noise exposure and pathogenic variants in the **Coch** allele

The main topic of this doctoral thesis is the COCH gene and the protein which it encodes, cochlin. This protein is mainly expressed in the spiral ligament of the inner ear. The function of cochlin is not fully understood but there is a growing body of evidence that it plays an important role in the innate immune system of the cochlea. Furthermore, cochlin has a role in maintaining the structure and function of the extracellular matrix of the inner ear. One of the disorders caused by mutations in the COCH gene is DFNA9, which is a dominant hereditary disorder characterized by progressive hearing loss and vestibular dysfunction. Different heterozygous mutations can cause DFNA9. Dependent on the exact location of the mutation in the COCH gene, a slightly different phenotype and pathology is observed between DFNA9 patients harbouring different pathogenic variants. A Coch knockout mouse model has been used to investigate the role of cochlin in the inner ear. These mice do not express cochlin as exon 7 to exon 12 of the COCH gene is removed. Both Coch knockout mice and Coch wildtype mice were exposed to noise in order to investigate the role of cochlin after noise exposure. The first section of this thesis focuses on the optimization of hearing and vestibular assessment in mice. To evaluate hearing function, ABR and DPOAE measurements were performed. These quantitative measurements assess inner and outer hair cell function respectively and require the mice to be under anaesthesia during the tests. A test battery based on observation of behaviour and different reflexes was performed to evaluate vestibular function semi-quantitatively in mice. Two different anaesthetics are commonly used to perform these measurements: isoflurane (an inhalation anaesthetic) and a mixture of ketamine and xylazine (a parenteral anaesthetic). In the first section of this PhD thesis, the effect of both anaesthesia methods on hearing thresholds in mice was evaluated. Furthermore, allylnitrile was administered to mice to obtain a total loss of hearing function. This allowed us to assess whether our set-up could differentiate between deaf mice and mice with a normal hearing function. The second part of this doctoral thesis describes the function and physiology of the spiral ligament fibrocytes. The spiral ligament forms the outer wall of the cochlea and consists of five different types of fibrocytes. Cochlin is an important protein in the spiral ligament and plays a key role in maintaining the ion homeostasis in the endolymph, the regulation of the cochlear blood flow and immune response in the cochlea. In addition, section two outlines the pathology observed in the spiral ligament of DFNA9 patients and the different pathogenic variants present in the COCH gene and their effect on hearing loss and vestibular dysfunction. In section three of this doctoral thesis, Coch knockout and Coch wildtype mice were exposed to noise. Noise exposure can induce hearing loss by causing damage to different cochlear cells. In addition, an inflammatory response and oxidative stress in the inner ear enhance hearing loss. The inferior region of the spiral ligament, where COCH expression is most abundant, is the most sensitive to noise exposure. Therefore, in the third section of this doctoral dissertation, investigating the role of cochlin after noise exposure was performed by exposing both Coch knockout and Coch wildtype mice to broadband noise. Finally, the fourth part of this dissertation focuses on the use of an adeno-associated viral vector to deliver gene therapy into the inner ear. Gene therapy can have the potential to prevent otovestibular loss originating from pathogenic variants in several genes. Even though AAV are the most frequently used viral vectors to deliver gene therapy into the cochlea, little is known regarding their transduction efficiency in different cochlear cell types and the possible induction of an immune response. This part gives an overview of the transduction efficiency of several AAV serotypes in different cochlear cell types as well as the best method to inject AAV in the inner ear, the optimal AAV concentration and the possible risk regarding an immune response in the cochlea.

Jaar van publicatie:2022

Trefwoorden:Doctoral thesis

Toegankelijkheid:Closed