Journal Club: Comparison of IHC synapses and OHC loss in ICR, C57/BL6 and CBA/CaJ mice with age.

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Today's journal article

Lankinen T, Leinonen S, Ikäheimo K, Pirvola U. Relationship between inner hair cell synaptopathy and outer hair cell loss in two mouse models of accelerated age-related hearing loss. 

Why I picked this article

Age-related sensorineural hearing loss occurs from damage to the inner ear organ for hearing, the cochlea, and the auditory nerve. From a sensory cell perspective, age-related hearing loss is often described as two coupled problems that happen in the cochlea: damage to outer hair cells (OHCs) that raises the hearing thresholds (hence quiet sounds cannot be heard), and a loss of inner hair cell (IHC) ribbon synapses, that when lost, the communication between IHCs and auditory neurons become reduced ( = cochlear synaptopathy). 

Studying the age-related sensorineural hearing loss has been challenging because age-related changes can be affected by additional factors like environmental noise and genetic makeup. In laboratory science, a certain strain of inbred mice will develop hearing loss earlier than other strains due to differences in their genetic makeup. This can cause a problem when we try to model age-related hearing loss using rodents to understand the pathology and to develop new treatments. 

In the classic CBA/CaJ mouse, synaptopathy typically precedes OHC loss, implying partially separate mechanisms. This research aims to investigate the independence/dependence of OHC loss and IHC synaptopathy in another additional mouse model, ICR mouse, and compare with C57BL/6 mice. 

Some of the research findings

Animal models: 
  • Mouse strains were C57BL/6-based and ICR: 
  • C57BL/6JRccHsd (Inotiv, # 043)
  • Hsd:ICR (CD-1®) (Inotiv, # 030)
  • CBA/CaJ (The Jackson Laboratory, # 000654)
  • Previous research showed that the hearing loss occurs at different timing in three strains: 
    • CBA/CaJ mice are white inbred mice which retain good hearing until about 20 months old, and are considered a good model for age-related hearing loss. Previous research shows that IHC synapse loss progresses earlier than OHC loss. 
    • C57/BL6 black mice, which show progressive hearing loss as early as 2-3 months old. Defect in OHCs and IHC synapse loss both occur early. 
    • ICR mice also develop hearing loss early. 
Outcome measures: 
  • Puretone auditory brainstem response (ABR), 4 to 45 kHz frequency. 
  • Cochleae collection and immunohistochemistry to visualise hair cells and synapses. 
  • FM1-43 fluorescence assay: 
    • FM1-43FX dye (Thermo Fisher Scientific, Cat # F35355) 5 μM concentration diluted in in Leibovitz’s L-15 medium, was injected into the cochlea. 
    • FM1-43 was pulsated through the round and oval windows for 90 seconds. 
    • Cochleae were fixed and imaged by microscopy. 
    • The dye uptake is observed as a sign of actively firing neurons. In this study, FM1-43 dye was used to label IHCs with active stereocilia and MET-channel opening (i.e. actively undertaking sound transduction). 
Key findings: 
  • C57/BL6 mice exhibited significant OHC loss and IHC synapse loss at 6-8 month-old, more severe in high-frequency zones.
  • ICR mice exhibited significant OHC loss and IHC synapse loss at 9 weeks, with some changes evident at 5-week-old. 
  • Across high→low frequency regions, regression analyses showed a strong correlation between IHC synapse counts and OHC survival: regions with fewer IHC synapses also had more OHC loss in both C57/BL6 and ICR mice groups.
  • Using FM1-43 dye uptake as a proxy for MET channel function, IHCs were labelled less with FM1-43 in cells which had fewer synapses. 
  • It suggests that IHCs with synaptopathy also showed MET malfunction. This is consistent with the fact that strains carrying a missense mutation in Cadherin 23 (Cdh23), a tip-link/bundle protein. 
  • FM1-43 labelling appeared less affected in CBA/Caj mice and was abolished in the negative control (BAPTA-treatment). 
  • TUDCA (a bile-acid derivative that modulates ER/mitochondrial stress pathways) is a compound previously shown to contrl stress signalling. 
  • TUDCA was injected twice per week from 3-9 weeks of age in ICR mice. 
  • TUDCA administration in ICR mice partially protected IHC synapses but did not prevent OHC loss. 
Researchers suggest that in ICR and C57/BL6 mice, stereocilia/tip-link stress → MET impairment → IHC synaptopathy; in parallel, stereocilia stress in OHCs contributes to OHC death, and that stress-pathway modulation (TUDCA) may support synaptic resilience without fully rescuing OHC survival. 

From part of Fig.5E, showing inner hair cells with low FM1-43 dye uptake (arrowheads, top panel) have fewer synapses (Ctbp2, bottom panel), ICR mice at 7 weeks old. Lankinen et al. (2025). 

Haruna's takeaway

This research provides a very useful baseline of data on age-related pathologies in the mouse strains. ICR mice are white mice that give a large number of pups, so they are very handy to use for some research. I didn't know that they had such early-onset hearing loss. Checking IHCs from both synapses and MET-channel function (dye uptake) seems very useful to understand the status of IHCs, rather than just counting their survival. 

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This is Haruna's 99/100 of the 100-day challenge to post a science blog article every day! I love inner ear biology & cochlear physiology.