Age-dependent modifications of expression level of VEGF and its receptors in the inner ear

The mechanisms responsible for age-associated hearing loss are still incompletely characterized. In this study, we used a murine model of age-dependent hearing loss and evaluated whether this condition is associated with vascular modifications of the structures of the inner ear. We used old C57BL/6J mice that are affected by rapid and severe age-related hearing loss, and analyzed the expression pattern of vascular endothelial growth factor (VEGF), a prototypical angiogenic cytokine, and its receptors Flt-1 and Flk-1 in the inner ear. We report for the first time morphological and quantitative data about the expression of these crucial angiogenic molecules in the murine cochlea. We also show that in this animal model, cochlear VEGF expression is significantly reduced as a function of age. Our findings provide new evidence of possible interdependent relationships between aging, VEGF, and presbycusis, suggesting that vascular abnormalities might play a role in aging-associated hearing loss, with potentially important fundamental and clinical implications.

Age-related hearing loss and its association with reactive oxygen species and mitochondrial DNA damage

Age-related hearing loss, known as presbyacusis, is characterized by the progressive deterioration of auditory sensitivity associated with the aging process and is the leading cause of adult auditory deficiency in the USA. Presbyacusis is described as a progressive, bilateral, high-frequency hearing loss that is manifested on audiometric assessment by a moderately sloping pure tone audiogram. Approximately 23% of the population between 65 and 75 years of age, and 40% of the population older than 75 years of age are affected by this condition. It was estimated in 1980 that 11% of the population was 76 years or older and this number is expected to almost double by the year 2030. When one considers that the population over 65 years of age is experiencing the most accelerated development of hearing loss, the potential socioeconomic ramifications are staggering. Curiously, the frequency of presbyacusis varies across different societies. This discrepancy has been attributed to many factors including genetics, diet, socioeconomic factors, and environmental variables. The purpose of this article is to review the various molecular mechanisms underlying presbyacusis and to offer insights into potential methods of mitigating the effects of aging on hearing impairment.

Correlation of cochlear blood supply with mitochondrial DNA common deletion in presbyacusis

OBJECTIVE

To study the relationships between cochlear hypoxia, mitochondrial (mt) DNA4977 deletion and metabolic features of mtDNA in presbyacusis.

MATERIAL AND METHODS

Sixty-seven temporal bones from a presbyacusis group, an age-matched control group and a young and middle-aged control group were involved in the experiment. Nested and tri-nested polymerase chain reactions (PCRs) were applied to test for the presence of the mtDNA4977 deletion. Computer imaging processing was used to measure blood vessel parameters in the internal acoustic meatus (IAM).

RESULTS

The mtDNA4977 deletion was detected in 17/34 ears in the presbyacusis group, 4/19 ears in the age-matched control group and 0/14 ears in the young and middle-aged control group. In the presbyacusis group, the lumen of the vasa nervorum of the IAM showed a more severe narrowing in cases with than without the mtDNA4977 deletion.

CONCLUSION

The high incidence of the mtDNA4977 deletion in the temporal bones of presbyacusis patients suggests a correlation between the mtDNA4977 deletion and presbyacusis. Hypoxia of the cochlea may cause the mtDNA4977 deletion and other mtDNA mutants and furthermore may cause a reduction in mitochondrial oxidative phosphorylation and decreased function of the acoustic neural system. The symptoms of presbyacusis may occur when the function of the acoustic neural system is impaired as a result of abnormal mtDNA metabolism reaching a particular threshold.

Mutation in Mitochondrial DNA as a Cause of Presbyacusis

Much of the hearing loss that occurs in old age is likely to be due to the long-term deterioration of the mitochondria in the different structures of the cochlea. The current review surveys some of the basic information on mitochondria and mitochondrial DNA, as a background to their possible involvement in presbyacusis. It is likely that oxygen radicals damage mitochondrial DNA and other components of the mitochondria, such as their proteins and lipids. This further compromises both oxidative phosphorylation and the repair processes in mitochondria, setting up a vicious cycle of degradation. Evidence is presented from inherited point mutations on the possibly most critical sites for mutations in mitochondrial DNA associated with hearing loss. It is suggested that random sorting and clonal expansion of mutations both maintain the integrity of the pool of mitochondrial DNA molecules and give rise to the apoptosis that leads to loss of vulnerable cells, and hence to deafness. It is moreover suggested that apoptosis of the vulnerable cells of the inner ear may to some extent be preventable, or at least delayed.

Metabolic presbycusis: differential changes in auditory brainstem and otoacoustic emission responses with chronic furosemide application in the gerbil

Auditory characteristics of metabolic or strial presbycusis were investigated using an animal model in which young adult Mongolian gerbils ( Meriones unguiculates) were implanted with an osmotic pump supplying furosemide continuously to the round window. This model causes chronic lowering of the endocochlear potential (EP) and results in auditory responses very similar to those seen in quiet-aged gerbils (Schmiedt et al., J. Neurosci. 22:9643-9650, 2002). Auditory function was examined up to one week post-implant by measurement of auditory brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs). Emission "threshold" was defined as the stimulus level required to reach a criterion emission amplitude. Comparing all responses on a "threshold-shift diagram," where emission threshold increases were plotted versus ABR threshold increases, the following results were obtained: (1) On average, the increase of the emission threshold was about 55% of the increase in ABR threshold, with comparatively little scatter. (2) The main dysfunction in metabolic presbycusis appears to be a decrease in the gain of the cochlear amplifier, combined with an additional, smaller increase in neural threshold, both effects caused by a chronically low EP. (3) For ABR threshold increases over 20 dB, the points for the chronic low-EP condition were largely separate from those previously found for permanent acoustic damage. The threshold-shift diagram therefore provides a method for noninvasive differential diagnosis of two common hearing dysfunctions.

Auditory peripheral influences on calcium binding protein immunoreactivity in the cochlear nucleus during aging in the C57BL/6J mouse

The C57BL/6J (C57) mouse was selected as a suitable model for early presbyacusis to determine if there were correlations between peripheral pathology (spiral ganglion loss, inner and outer hair cell loss) and calcium binding immunoreactivity in the cochlear nucleus during aging. The quantitative stereological method, the optical fractionator, was used for determining the total number of neurons and calcium binding immunopositive neurons (calbindin, parvalbumin and calretinin) during aging in the posteroventral- and dorsal cochlear nucleus (PVCN and DCN) in C57 mice. Comparing 30-month-old to 1-month-old C57 mice, a percent increase in parvalbumin and calbindin immunoreactivity was evident in both the PVCN and DCN. Correlations were made between peripheral pathology (spiral ganglion and inner and outer hair cell loss) and calcium binding protein expression. Significant correlations between cochlear pathology and the percentage of parvalbumin and calretinin immunoreactive neurons were demonstrated in the DCN. Moreover, significant correlations were found between cochlear pathology and parvalbumin and calbindin in the PVCN. In summary, the findings imply that degenerative changes in the auditory periphery can modulate neuronal homeostasis by increasing calcium binding proteins in the PVCN and DCN during aging. Taken together, these findings suggest a role for calcium binding proteins in protecting against age-induced calcium toxicity.

Effects of furosemide applied chronically to the round window: a model of metabolic presbyacusis

Hearing thresholds in elderly humans without a history of noise exposure commonly show a profile of a flat loss at low frequencies coupled with a loss that increases with frequency above approximately 2 kHz. This profile and the relatively robust distortion product otoacoustic emissions that are found in elderly subjects challenge the common belief that age-related hearing loss (presbyacusis) is based primarily on sensory-cell disorders. Here, we examine a model of presbyacusis wherein the endocochlear potential (EP) is reduced by means of furosemide applied chronically to one cochlea of a young gerbil. The model results in an EP that is reduced from 90 to approximately 60 mV, a value often seen in quiet-aged gerbils, with no concomitant loss of hair cells. Resulting measures of cochlear and neural function are quantitatively similar to those seen in aging gerbils and humans, e.g., a flat threshold loss at low frequencies with a high-frequency roll-off of approximately -8.4 dB/octave. The effect of the EP on neural thresholds can be parsimoniously explained by the known gain characteristics of the cochlear amplifier as a function of cochlear location: in the apex, amplification is limited to approximately 20 dB, whereas in the base, the gain can be as high as 60 dB. At high frequencies, amplification is directly proportional to the EP on an approximately 1 dB/mV basis. This model suggests that the primary factor in true age-related hearing loss is an energy-starved cochlear amplifier that results in a specific audiogram profile.

Spiral ligament pathology in quiet-aged gerbils

The ultrastructure of the spiral ligament was compared in aged and young gerbils to assess the involvement of connective tissues in the lateral wall and particularly the fibrocytes in development of presbyacusis. Pathologic features in fibrocytes of senescent gerbils spanned a wide range reflecting different stages of lateral wall involution. All of the type II, IV and V fibrocytes selectively developed cytosolic vacuoles in an early degenerative phase showing minimal strial involvement. Clear spaces indicative of interstitial edema separated the vacuolated cell bodies and their plasmalemmal processes. As a presumed intermediate phase, profiles of amorphous substance apparently derived from apoptosis/necrosis of type II fibrocytes infiltrated the type II fibrocyte area among nearly normal appearing cells. In cochlear turns with advanced strial degeneration, type II fibrocytes disappeared from the spiral prominence area leaving only type I-like fibrocytes occasionally accompanied by a collagen infiltrate. Type V fibrocytes disappeared similarly from the suprastrial area. The extent of atrophy in type II fibrocytes corresponded in general with that in the neighboring stria vascularis. Age-dependent atrophy in the lateral wall largely spared type I fibrocytes except that they often enclosed discrete amorphous foci lacking organelles. The involution thus affected principally the Na,K-ATPase-positive fibrocytes functioning in active uptake rather than passive conductance of K(+). The vacuolization and degeneration exclusive to ATPase-rich fibrocytes and the associated intercellular edema are interpreted as secondary responses, possibly as a result of impaired diffusion of K(+) through downstream marginal cells.

Expression of Klotho protein in the inner ear

The Klotho mouse is a recently developed model that exhibits phenotypes resembling human aging. We used this model to investigate sensorineural hearing loss from the point of view that it may be considered an issue of aging. Using reverse transcription-polymerase chain reaction, Western blotting and immunohistochemical staining, we were able to confirm klotho gene transcription and protein synthesis in the kidney and inner ear. Klotho protein was mainly expressed in the stria vascularis and spiral ligament of the inner ear and in the distal convoluted tubule of the kidney, likely serving a common function in the two organs, i.e., modulating ion transport. The threshold for the auditory brainstem response was significantly higher in Klotho mice than in wild-type mice, and wave I latencies were prolonged. On the other hand, Klotho mice exhibited a normal distribution of I-IV interpeak intervals. No obvious morphological abnormalities were detected in Klotho mice, although no expression of Klotho protein was detected, and there was an apparent hearing disorder. Taken together, these findings suggest that by contributing to the maintenance ion homeostasis in the endolymph, Klotho protein serves as a key mediator of auditory function.

Morphology, GluR1 and GRIP-C localization differ in octopus cells of C57BL6 and B6Cast mice

The C57BL6 mouse (B6) is homozygous for the gene for age-related hearing loss (ahl/ahl) and shows normal adult-like hearing before subtle changes in hearing begin at about 30 days of age. The B6Cast mouse is congenic to B6, having the wild type allele for normal hearing from Castaneous Ei on a B6 background. It has normal hearing throughout most of its lifespan. This study characterized the morphology of octopus cell (OC) somata in the posterior-ventral cochlear nucleus and of synaptic terminals on the OC somata in 8-week-old B6 and B6Cast mice, and the immunolocalization of antibodies to GluR1 (glutamate receptor subunit 1) and GRIP-C (glutamate receptor interacting protein-C terminus). By 8 weeks of age there are significant changes in the morphology of OCs and synaptic terminals around their somata in B6 mice compared to B6Cast mice. The distribution of immunoreactivity for the proteins GluR1 and GRIP is also significantly different in B6 mice from that in B6Cast mice. The modest degenerative changes reported in some B6 outer hair cells of the basal turn at this age do not seem adequate to explain the major changes observed in most OCs at a time when physiological studies show that many measures of the animals' hearing are still near normal. The findings suggest that changes in the alpha-amino-3-hydroxy-5-methyl-4-isoxazole glutamate receptor subunits and/or their binding proteins are part of the phenotype of ahl, and may reflect a role of the glutamate receptor pathway in the mechanism of ahl.

Estrogen receptors alpha and beta in the inner ear of the 'Turner mouse' and an estrogen receptor beta knockout mouse

Estrogen receptors have earlier been shown in the normal mouse, rat and human inner ear. If estrogens are important in normal hearing and development of presbyacusis in the normal population is not known. However it is known that patients with Turner syndrome, where a lack of estrogens is one of the main characteristics, commonly develop an early presbyacusis. A 'Turner mouse' has been developed, as a model for the ear problems in Turner syndrome, and it shows otitis media and a premature aging of the hearing. Estrogen receptors exist in an alpha and a beta form. In this study inner ear tissue, from the Turner mouse and an estrogen receptor beta knockout mouse (betaERKO), was investigated regarding estrogen receptor alpha and beta using immunohistochemistry. Results show that the Turner mouse has the same pattern of inner ear labeling, both concerning the estrogen receptor alpha and beta, as that of a normal CBA/Ca mouse, with positive staining in the organ of Corti and spiral ganglion. The betaERKO mice show close to normal inner ear morphology and positive estrogen receptor alpha immunostaining at the same locations as the CBA/Ca mouse.

Effects of aging on C57BL/6J mice: an electrophysiological and morphological study

Presbycusis is a progressive hearing loss associated with aging that manifests as deafness linked to cochlear morphological degeneration. The effects of aging on the auditory system were studied in C57BL/6J mice using electrophysiological (brainstem auditory evoked potentials; BAEP) and morphological techniques. Cochleae of animals aged 1, 6, 9, 12, 15, 18, 21, or 24 months old were used for that purpose. The BAEP showed a progressive increase in latency and a reduction in amplitude. Morphological studies demonstrated total degeneration of the organ of Corti, which was replaced by a single epithelial layer. An affinity histochemistry study demonstrated minor modifications of glycoconjugates in the organ of Corti during the aging process.

Clinical experience with caroverine in inner ear diseases

The glutamatergic synapses between the cochlear inner hair cells and their afferent neurons seem to be mostly involved in the pathophysiology of the cochlea. Glutamatergic neurotoxicity is characterized by a mitochondrial overproduction of free oxygen radicals damaging lipid membranes and DNA structures of the postsynaptic neuron followed by the clinical symptoms of hearing loss and tinnitus. In preclinical tests, quinoxaline derivatives antagonized these deleterious consequences of too high an amount of free radicals. Therefore the clinically available quinoxaline dione caroverine provides a new approach to a successful treatment of tinnitus, sudden hearing loss and speech discrimination disorders in presbyacusis. The results of corresponding clinical trials are presented.

Galectine-1 expression in cochleae of C57BL/6 mice during aging

Presbycusis is a progressive hearing loss related to the aging process associated with auditory receptor degeneration. Adult animals exhibited galectine-1 (GAL-1) expression in epithelial cells of the inner and outer spiral sulci and Hensen cells, which was absent from the sensory and supporting cells. The progressive age-related degeneration of auditory receptor was analyzed using an anti-GAL-1 monoclonal antibody on cochleae of C57BL/6 mice (1-24 months old). Mice < or = 9 months old showed adult and healthy auditory receptors, with a similar GAL-1 expression along the time. Conversely, animals > 1 year old exhibited a sensory degeneration at the cochleae basal coil that progressively reached the middle coil. Older animals (18-24 months old) showed a single layer epithelium that replaced the auditory receptor. The age-related degenerative process of the auditory receptor involves sensory but also supporting cells, which are both substituted by epithelial non-specialized cells.

Biologic activity of mitochondrial metabolites on aging and age-related hearing loss

HYPOTHESIS

Compounds that upregulate mitochondrial function in an aging model will improve hearing and reduce some of the effects of aging.

BACKGROUND

Reactive oxygen metabolites (ROM) are known products of oxidative metabolism and are continuously generated in vivo. More than 100 human clinical conditions have been associated with ROM, including atherosclerosis, arthritis, autoimmune diseases, cancers, heart disease, cerebrovascular accidents, and aging. The ROM are extremely reactive and cause extensive DNA, cellular, and tissue damage. Specific deletions within the mitochondrial DNA (mtDNA) occur with increasing frequency in age and presbyacusis. These deletions are the result of chronic exposure to ROM. When enough mtDNA damage accrues, the cell becomes bioenergetically deficient. This mechanism is the basis of the mitochondrial clock theory of aging, also known as the membrane hypothesis of aging. Nutritional compounds have been identified that enhance mitochondrial function and reverse several age-related processes. It is the purpose of this article to describe the effects of two mitochondrial metabolites, alpha-lipoic acid and acetyl L-carnitine, on the preservation of age-related hearing loss.

METHODS

Twenty-one Fischer rats, aged 24 months, were divided into three groups: acetyl-l-carnitine, alpha-lipoic acid, and control. The subjects were orally supplemented with either a placebo or one of the two nutritional compounds for 6 weeks. Auditory brainstem response testing was used to obtain baseline and posttreatment hearing thresholds. Cochlear, brain, and skeletal muscle tissues were obtained to assess for mtDNA mutations.

RESULTS

The control group demonstrated an expected age-associated threshold deterioration of 3 to 7 dB in the 6-week study. The treated subjects experienced a delay in progression of hearing loss. Acetyl-l-carnitine improved auditory thresholds during the same time period (p<0.05). The mtDNA deletions associated with aging and presbyacusis were reduced in the treated groups in comparison with controls.

CONCLUSIONS

These results indicate that in the proposed decline in mitochondrial function with age, senescence may be delayed by treatment with mitochondrial metabolites. Acetyl-l-carnitine and alpha-lipoic acid reduce age-associated deterioration in auditory sensitivity and improve cochlear function. This effect appears to be related to the mitochondrial metabolite ability to protect and repair age-induced cochlear mtDNA damage, thereby upregulating mitochondrial function and improving energy-producing capabilities.

Mechanisms of alterations in the microcirculation of the cochlea

Labyrinthine function is tightly coupled to proper homeostasis. This includes appropriate blood flow that is under strict autoregulatory control. Perturbations in labyrinthine microcirculation can lead to significant cochlear and vestibular dysfunction. The etiology of many otologic disorders, including sudden sensorineural hearing loss, presbyacusis, noise-induced hearing loss, and certain vestibulopathies, are suspected of being related to alterations in blood flow. Some of the mechanisms responsible for hypoperfusion and possibly ischemia, within the cochlea, are addressed, with emphasis on the possibility that both noise and age contribute to localized low blood-flow states and stasis. This reduction in blood supply to the cochlea is likely, in part, responsible for reduced auditory sensitivity associated with chronic noise exposure and aging.

Aging does not alter phosphoinositide hydrolysis in the rat cochlear lateral wall

We have previously reported that the inositol 1,4,5-trisphosphate (InsP3) second messenger system is coupled to purinergic P2y receptors in the cochlear sensory epithelium and lateral wall. The tissues of the cochlear lateral wall (stria vascularis and spiral ligament) are responsible for maintaining the ionic composition of the cochlear endolymph. Both the endolymphatic potential and signal transduction processes are well known to be affected by aging. Furthermore, intracellular inositol concentrations decrease with age in the cochlear sensory epithelia. The present study compared the purinergic receptor-mediated release of inositol phosphates (InsPs) in the cochlear lateral wall of young (3 month-old) and aged (24 month-old) Fischer-344 rats. No differences were found in the incorporation of mnyo-[3H]inositol into phosphoinositide lipids. Likewise, the purinergic receptor-mediated release of InsPs remained unchanged. This suggests that the InsPs second messenger system in the cochlear lateral wall, in contrast to the sensory epithelium, may not be affected by aging.

[Changes of GABA immunoreactivity in aged rat cochlear nucleus]

OBJECTIVES

To detect the changes of gama-aminobutyric acid (GABA) immunoreactivity in the aged rat cochlear nucleus and to investigate its relationship with the initiation of presbycusis.

METHODS

The cochlear nucleuses of adult (2-3 months) and aged (20-36 months) Wistar rats were stained with streptavidin/peroxidase (S-P) technique. GABA-positive neurons were counted with a computer-aid image analysis system.

RESULTS

Most of the GABA-immunoreactive cell bodies were found in the superficial layers of the dorsal cochlear nucleus (DCN). The number of GABA-positive neurons decreased significantly in the DCN of the aged rat cochlear nucleus (P < 0.001).

CONCLUSION

The reduction of GABA positive neurons in the aged rat cochlear nucleus may be related to presbycusis.

Calbindin D-28k immunoreactivity in the medial nucleus of the trapezoid body declines with age in C57BL/6, but not CBA/CaJ, mice

This study compared calbindin D-28k immunoreactivity in the medial nucleus of the trapezoid body (MNTB) in young (3-4 month old) and old (24-26 month old) CBA/CaJ mice, and young (3-4 month old), middle-aged (6.5-8.5 month old), and old (24-29 month old) C57BL/6 mice. C57BL/6 mice exhibit progressively more severe peripheral (sensorineural) hearing loss between 4 and 12 months of age, whereas CBA/CaJ mice show little change in peripheral sensitivity until very late in life. We obtained auditory brainstem response audiograms on all subject mice. Old CBA mice were selected for study whose audiograms matched those of young CBA and C57 controls. Middle-aged C57 mice showed elevated thresholds indicative of peripheral degeneration. Brain sections were reacted with anti-calbindin D-28k (CB). Staining patterns in Nissl and anti-CB material were characterized and cells were counted. We found no significant change in the number of CB+ cells or the total number of cells in the MNTB of old CBA mice compared to young controls. However, the mean number of CB+ cells decreased by 11% in middle-aged, and by 14.8% in old C57 mice. Since the decline in C57 mice was significant by 6.5-8.5 months of age, the decrease could be the consequence of a loss of input from the cochlear nucleus where cell numbers are known to decline by this age in this strain. The total number of neurons in MNTB assessed from Nissl material showed a more modest 7.1% decline with age in C57 mice, implying that the greater loss of CB immunoreactive cells with age cannot be completely attributed to a reduction in the total number of cells.

Aging and presbycusis: effects on 2-deoxy-D-glucose uptake in the mouse auditory brain stem in quiet

Autoradiography was used to assess the incorporation of [2-14C]deoxy-D-glucose by the auditory brain stem of young and aging mice of the C57BL/6 strain (which demonstrates progressive chronic sensorineural hearing loss) and the CBA strain (which maintains good hearing until late in life). Animals were injected with labeled 2-deoxyglucose and placed in quiet for 45 min; brain stem sections were prepared for autoradiography. The amounts of 2-deoxyglucose incorporated into the anterior ventral cochlear nucleus (AVCN), inferior colliculus (IC), and trigeminal nerve (TN) were densitometrically analyzed. Within each subject, the densities of the three structures were statistically compared. In every mouse, inferior colliculus density was greater than that of the anterior ventral cochlear nucleus, which was greater than trigeminal nerve density. To compare subject groups, relative densities (inferior colliculus and anterior ventral cochlear nucleus re: trigeminal nerve) were used; no significant differences were found between groups. Thus, aging, with or without severe loss of hearing, is not associated with altered incorporation of 2-deoxyglucose (and presumably glucose) in quiet.