Hearing is initiated when sound-induced vibrations of the eardrum and middle-ear ossicles are transmitted to the fluids of the inner ear, leading to stimulation of sensory hair cells and excitation of the auditory nerve. Hearing loss is broadly classified into two categories: (1) conductive hearing loss (CHL), in which mechanical energy transfer from the air to the inner ear is impeded, and (2) sensorineural hearing loss (SNHL), in which tissue pathology in the inner ear or central auditory pathways hamper signal transduction or neural conduction. SNHL can be the result of damage to almost any of the inner ear’s approximately 30 different cell types, as well as the nerves connecting the cochlea to the brain. Although SNHL can be routinely differentiated from CHL, diagnosing the specific pathology invoking an individual patient’s SNHL is a major challenge. No method currently exists to directly evaluate the inner ear, leaving the clinician blind to the underlying pathologic condition in SNHL. Indirect measures, such as audiograms, word recognition scores1
, auditory brainstem responses and otoacoustic emissions2
can provide some insight, but are unable to make a specific determination.
Clinically, the limitation described above results in a frequent diagnosis of idiopathic SNHL leaving the patient and physician with no clear prognosis and general, and often ineffective, treatment options. A diagnostic tool capable of providing insight into inner ear pathology would enable the formulation of individualized treatment strategies (“personalized medicine”) tailored to each patient’s specific pathology. The need for such a tool is immediately apparent from the large number of patients who have unsatisfactory results with general treatment options. Furthermore, a diagnostic platform is critical for successful implementation of preservative and restorative therapies that may emerge from ongoing research in inner ear development and regeneration3
. Positive patient outcomes will only result if specific disease states are known and targeted, making a diagnostic tool absolutely critical to treatment success.
To lay the groundwork for development of a diagnostic platform capable of filling this clinical need, we present the first analysis of the human perilymph proteome using mass spectrometric (MS) techniques. Perilymph, a proximal fluid of the inner ear, bathes spiral ganglion cell bodies of the auditory nerve and nearly all of the tissues vital to sound transduction. Due to its localization, any protein secreted by a damaged cell or released during an apoptotic or necrotic event will be found in perilymph at higher concentrations than in more peripheral fluids such as blood or cerebrospinal fluid (CSF).
Previous work has demonstrated the utility of perilymph as a diagnostic fluid. Prior to modern imaging technologies, vestibular schwannoma (VS), an important cause of SNHL in clinical practice, was diagnosed via a significant, i.e. >2.5-fold increase in the total protein content of perilymph4,5
. This example demonstrates the potential for significant change in perilymphatic protein levels as a result of pathology, encouraging our search for relevant diagnostic information. Identification and validation of a specific set of biomarkers coupled with the development of a refined collection technique, minimizing the risk to hearing, may facilitate the collection and analysis of perilymph for diagnostic purposes in a clinical setting.
The present investigation is, to the best of our knowledge, the first attempt to define the proteome of human perilymph using mass spectrometry. Previous knowledge of the perilymphatic proteome is derived mainly from 2-D gel electrophoresis work aimed at diagnosis of perilymphatic fistula6–9
. These early investigations established the presence of over 100 proteins in the fluid, nearly 30 of which were subsequently identified. Our work extends the existing proteomic characterization and compares the protein profile of perilymph to other bodily fluids. We have compared protein content in different pathologic states as a first step towards the discovery of disease biomarkers. The similarity of human and mouse perilymph has also been analyzed to explore the potential applicability of mouse models for discovery of biomarkers relevant for human SNHL.
Beyond the previously mentioned diagnostic possibilities, the knowledge generated in this investigation will be of significant utility to basic auditory scientists and inner ear pharmacologists. Characterization of the protein content of perilymph may provide insight into the molecular mechanisms that function to maintain the inner ear’s unique environment. Knowledge of the fluid content will also allow better understanding and prediction of protein-drug interactions, aiding in assessment of pharmacological efficacy and drug delivery within the highly specialized organ.