Annexin A5 is the Most Abundant Membrane-Associated Protein in Stereocilia but is Dispensable for Hair-Bundle Development and Function

The phospholipid- and Ca²⁺ -binding protein annexin A5 (ANXA5) is the most abundant membrane-associated protein of ∼P23 mouse vestibular hair bundles, the inner ear's sensory organelle. Using quantitative mass spectrometry, we estimated that ANXA5 accounts for ∼15,000 copies per stereocilium, or ∼2% of the total protein there. Although seven other annexin genes are expressed in mouse utricles, mass spectrometry showed that none were present at levels near ANXA5 in bundles and none were upregulated in stereocilia of Anxa5^-/- mice. Annexins have been proposed to mediate Ca²⁺ -dependent repair of membrane lesions, which could be part of the repair mechanism in hair cells after noise damage. Nevertheless, mature Anxa5^-/- mice not only have normal hearing and balance function, but following noise exposure, they are identical to wild-type mice in their temporary or permanent changes in hearing sensitivity. We suggest that despite the unusually high levels of ANXA5 in bundles, it does not play a role in the bundle's key function, mechanotransduction, at least until after two months of age in the cochlea and six months of age in the vestibular system. These results reinforce the lack of correlation between abundance of a protein in a specific compartment or cellular structure and its functional significance.