Relative Time Course of Degeneration of Different

Authors: Shanthini Mahendrasingam Jamie A MacDonald David N Furness

Publish Date: 2011/03/12

Volume: 12, Issue: 4, Pages: 437-453

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Abstract

Presbycusis agerelated hearing loss can result from various cochlear pathologies We have studied the time course of degeneration in a mouse that shows accelerated presbycusis the CD/1 mouse as a possible model to investigate stemcell strategies to prevent or ameliorate presbycusic changes CD/1 mice from 0 to 72 weeks old were examined by light and electron microscopy Early pathological changes were detected in basal turn spiral ligament fibrocytes and spiral ganglion but the latter was variable as both satellite cells and neurons were normal in some cochleae Light microscopic counts in the spiral ligament of 20weekold mice revealed that of the five main types types I–V only type V fibrocytes showed no reduction in numbers compared with 3weekold animals and type IV showed the greatest losses However all types of fibrocyte showed subtle damage when examined using electron microscopy in the form of swollen mitochondria as early as 2 weeks The extent of mitochondrial damage showed a degree of correspondence with the light microscopic pattern of fibrocyte loss in that types III and IV fibrocytes had the most abnormal mitochondria and type V the least especially at early stages By 10–15 weeks ultrastructural features of fibrocyte damage were similar to longer term changes reported in gerbils Stria vascularis spiral ganglion and hair cells showed few consistent early signs of damage but became increasingly affected lagging behind the fibrocyte damage Our data suggest that fibrocyte pathology may precede other presbycusic changes breakdown of homeostatic mechanisms to which they contribute may cause the subsequent degeneration of the hair cells Overall there were many similarities to presbycusic changes in other rodents and humans Therefore the features of accelerated aging in this mouse make it a suitable model for rapidly assessing possible strategies to prevent or ameliorate presbycusic changesThere are four main forms of agerelated hearing loss presbycusis in humans three of which are sensory neural affecting primarily the hair cells and spiral ganglion SG and strial degeneration Schuknecht and Gacek 1993 where the lateral wall stria vascularis SV and spiral ligament SL deteriorates In lateral wall degeneration the SL degenerates before the SV Kusunoki et al 2004 or hair cellsThe development of stemcell technology offers a means to repair the damaged or presbycusic cochlea Hildebrand et al 2008 or prevent presbycusis with timely intervention The outbred CD/1 mouse strain commonly shows accelerated presbycusis Shone et al 1991 with hearing loss at about 4 weeks prior to hair cell loss Le Calvez et al 1998a b Studies of these mice have shown different possible causes one suggests that SL damage precedes hair cell loss initially in basal turn with a reduction in endolymphatic K+ concentration Wu and Marcus 2003 However others report that the SG degenerates first Riva et al 2005 2007 These differences may reflect strain variations in different facilitiesThe SG consists primarily of afferent neuronal cell bodies with satellite cells surrounding them The satellite cells express proteins indicative of oxidative stress mitochondrial defects and apoptosis by 4 weeks of age Donadieu et al 2007 Riva et al 2007The SL consists of five main types of fibrocyte embedded in extensive extracellular matrix in mice Furness et al 2009 and gerbil Spicer and Schulte 2002 Type I fibrocytes are lateral to the SV type II lateral to the spiral prominence epithelium type III along the external edge of SL type IV in the basilar crest region and type V are suprastrial Spicer and Schulte 1991 2002 Nakazawa et al 1995 Ichimiya et al 2000 Suko et al 2000 Weber et al 2001 Fibrocyte degeneration occurs in various mouse strains Hequembourg and Liberman 2001 Wu and Marcus 2003 in labyrinthitis Ichimiya et al 1998 otitis media Ichimiya et al 1999 genetic hearing loss DFN3 Minowa et al 1999 noise exposure Hirose and Liberman 2003 and otospiralin knockouts Delprat et al 2005Mitochondrial degeneration in the SL as in the SG plays a crucial role in a mouse model of DFN3 fibrocytes had fewer mitochondria Minowa et al 1999 and in mice exposed to a mitochondrial toxin 3nitropropionic acid 3NP Okamoto et al 2005 SL pathology was accompanied by hearing loss Systemic administration of a caspase inhibitor prior to and during 3NP administration suggested that caspasedependent apoptosis of fibrocytes takes place as a result of mitochondrial damage Mizutari et al 2008The present study was motivated by the possibility of attempting SL cell replacement therapy using CD/1 mice bred in our animal facility We therefore have investigated the time course of degeneration of their cochlear structures specifically 1 the pattern of cell loss 2 the ultrastructural signs of early pathology and 3 the extent of mitochondrial damage using structural quantification and visualization of the cytochrome oxidase system Seligman et al 1968 1973 Perotti et al 1983CD/1 mice were bred and maintained in Keele University’s Central Animal Facility To avoid the strain from becoming significantly inbred stock was refreshed periodically by introducing new mice from the suppliers Charles Rivers All animals were treated in accordance with the UK Animals Scientific Procedures Act of 1986 and the project was given approval by Keele University’s ethical committee

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