- Title
- Ageing of the inner ear vestibular organs
- Creator
- Bigland, Mark James
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2019
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The inner ear peripheral vestibular apparatus contains the sensory organs that are critical to the detection of head movement, providing signals to control one’s head, eyes, and body, in three-dimensional space whilst under the influence of gravitational and other external forces. The signals generated by the vestibular neurosensory organs directly influence eye movements, the ability to walk, run, and any other form of movement that requires an individual’s gaze control and balance, and indirectly contributes to all aspects of motor function. Just as with other sensory systems, ageing has a negative influence on the vestibular system that culminates in age-related functional decline. Deficits in this system are often seen in the elderly as increased occurrences of dizziness, vertigo, disequilibrium, and falls, the latter being a prevalent cause of injury and morbidity in the elderly. These deficits contribute to a decrease in quality of life and are considered to be factors that can lead to premature death in the elderly. Increasingly, it is thought the functional decline in the vestibular peripheral apparatus cannot be attributed simply to a loss of the exquisitely sensitive neurosensory hair cells. Therefore, alternative mechanisms, which have been demonstrated in other sensory organs, may be contributing to the functional loss that manifests in this system. One such general mechanism of ageing is based on the age-related decline of mitochondrial function, mitochondria being the powerhouse of the cell, producing cellular ATP energy requirements. This decline is the basis for ‘the mitochondrial theory of ageing’, which posits that mitochondrial function declines with age due to an increasing burden of mitochondrial DNA (mtDNA) mutations, and/or increased production of reactive oxygen species (ROS). Chronic, low grade inflammation is another putative mechanism associated with ageing, which is known as the ‘inflammaging’ theory of ageing. In the nervous system other characteristics of ageing include perturbations to cholesterol metabolism and synapse function, with both of these being common features in many age-related neurodegenerative diseases. However, very little is known about any of these or other potential mechanisms that occur within the ageing peripheral vestibular organs. In Chapter 1, I used laser microdissection and genomics analyses to investigate the effects of ageing on the mitochondrial genome in vestibular hair cells and their supporting cells. My results indicated there was a decrease of approximately 50% in total mtDNA copy number in both cell subtypes, and this was substantially more than we had previously observed in aged rat brain tissue. Additionally, in contrast to our previous CNS findings in this rat strain, there was no increase in mtDNA deletion abundance with age, although there was a relative increase in abundance of mtDNA genes associated with the mitochondrial genome replication start site, potentially indicating abrogated mtDNA replication. Notably, if age-related abrogation of mtDNA replication occurred, it could explain the reduced mtDNA copy number. Furthermore, Gene Set Enrichment Analysis (GSEA) from preliminary microarray genome-wide gene expression studies revealed the oxidative phosphorylation (OXPHOS) pathway was attenuated with age. Overall, this study suggested a mtDNA-related impairment in ATP energy production with age, potentially causing the peripheral vestibular organs to be in energy crisis. In Chapter 2 a comprehensive genomics analysis of mouse vestibular organs at young adult, middle and old ages, revealed an up-regulation of genes and pathways involved in inflammatory processes and immune responses, with inflammatory indices being evident by middle age and increasing immune involvement between middle and old ages. The second half of the mouse life span was associated with a change to a ‘hostile microenvironment’ in the vestibular periphery, and this appeared to be consistent across the individual components of the vestibular periphery – the cristae, otoliths, and membranous labyrinth. This was also accompanied by alterations in expression of genes related to pathways in ion homeostasis and neuronal signalling. Overall, these genomics analyses revealed the mouse vestibular periphery developed an inflammaging-type phenotype, as is consistently seen in many other tissues and species with age. Additionally, there was an indication of altered neuronal signalling and function, which again is consistent with observations made in the ageing CNS. Chapter 3 builds on our novel and intriguing genomics finding that the gene encoding Cystic Fibrosis Transmembrane-conductance Regulator (CFTR) was up-regulated with ageing in the vestibular periphery. CFTR, a chloride channel, is the gene mutated in cystic fibrosis and was recently found to be expressed in cochlear hair cells, but to date, CFTR has not been the subject of study in vestibular end-organs. CFTR and its upregulation with age was confirmed in whole peripheral vestibular tissue. qPCR analyses showed CFTR mRNA is expressed in the cristae, otoliths, and membranous labyrinth. Increased mRNA occurred with ageing in both cristae and otoliths organs. Western blot of pooled young peripheral vestibular tissue revealed CFTR protein was indeed present. Electrophysiological hair cell patch clamp recordings, with and without pharmacological CFTR inhibition, demonstrated a variable response within young adult and aged vestibular hair cells, and therefore, further study is required to fully characterise the role of CFTR in the vestibular periphery. In Chapter 4, the effects of ageing on the cholinergic vestibular efferent system were investigated. Post-synaptic cholinergic signalling was shown to be affected in an age-related manner. Acetylcholine (ACh), a neurotransmitter that binds to α9-containing nicotinic ACh receptors (α9*nAChRs) in vestibular hair cells and triggers an influx of Ca2+ that activates nearby SK channels, leading to hair cell hyperpolarization. Patch clamp recordings in type II hair cells showed significantly reduced ACh evoked current amplitudes in old animals, a response that is consistent with the age-related reduction in gene expression for nicotinic cholinergic receptor subunits known to be critical for cholinergic responses in vestibular hair cells. These studies show ageing impairs cholinergic vestibular efferent system function, likely post-synaptically, and suggest this cholinergic hypofunction may contribute to age-related vestibular system dysfunction. In summary, my vestibular end-organ results demonstrated an age-related mitochondrial genomics profile indicative of a decline in mitochondrial energy production, suggesting the vestibular end organs may be on the brink of an energy crisis. These mitochondrial deficits were also accompanied by a marked increase in inflammatory signalling and immune response pathways, that were evident by middle age, as well as alterations in ion homeostasis (including CFTR gene expression), and a dysfunctional cholinergic efferent system. This places the aged peripheral vestibular organs in a state of diminished functional activity on many levels, and likely results in the debilitating symptoms so prevalent in the elderly.
- Subject
- ageing; vestibular; mtDNA; mitochondria; oxidative phosphorylation; hair cell; balance; thesis by publication
- Identifier
- http://hdl.handle.net/1959.13/1406131
- Identifier
- uon:35596
- Rights
- Copyright 2019 Mark James Bigland
- Language
- eng
- Full Text
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