Immunological characterization of two types of ionocytes in the inner ear epithelium of Pacific Chub Mackerel (Scomber japonicus)

• Published in: Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology Vol. 190; no. 4; pp. 419 - 431
• Main Authors: Kwan, Garfield T., Smith, Taylor R., Tresguerres, Martin
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.07.2020; Springer Nature B.V
• Subjects: Acidification; Adenylate cyclase; Animal Physiology; Biochemistry; Biomedical and Life Sciences; Biomedicine; Ca2+-transporting ATPase; Calcification; Calcium carbonate; Calcium homeostasis; Calcium ions; Calcium transport; Capillaries; Carbonic anhydrase; Carbonic anhydrases; Ear; Endolymph; Environmental stress; Epithelium; Evolutionary conservation; H+-transporting ATPase; Homeostasis; Human Physiology; Hydrogen; Immunohistochemistry; Immunology; Inner ear; Ion transport; Life Sciences; Localization; Mackerel; Marine fish; Membranes; Na+/Ca2+ exchanger; Na+/K+-exchanging ATPase; Ocean acidification; Original Paper; Physiology; Plasma; Polycrystals; Prey; Protein transport; Proteins; Saccule; Scomber japonicus; Trout; Western blotting; Zoology; Biomineralization; Calcification; Soluble adenylyl cyclase; Ocean acidification; ATPase; Otolith
• ISSN: 0174-1578; 1432-136X
• DOI: 10.1007/s00360-020-01276-3

The inner ear is essential for maintaining balance and hearing predator and prey in the environment. Each inner ear contains three CaCO 3 otolith polycrystals, which are calcified within an alkaline, K + -rich endolymph secreted by the surrounding epithelium. However, the underlying cellular mechanisms are poorly understood, especially in marine fish. Here, we investigated the presence and cellular localization of several ion-transporting proteins within the saccular epithelium of the Pacific Chub Mackerel ( Scomber japonicus ). Western blotting revealed the presence of Na + /K + -ATPase (NKA), carbonic anhydrase (CA), Na + -K + -2Cl − -co-transporter (NKCC), vacuolar-type H + -ATPase (VHA), plasma membrane Ca 2+ ATPase (PMCA), and soluble adenylyl cyclase (sAC). Immunohistochemistry analysis identified two distinct ionocytes types in the saccular epithelium: Type-I ionocytes were mitochondrion-rich and abundantly expressed NKA and NKCC in their basolateral membrane, indicating a role in secreting K + into the endolymph. On the other hand, Type-II ionocytes were enriched in cytoplasmic CA and VHA, suggesting they help transport HCO 3 − into the endolymph and remove H + . In addition, both types of ionocytes expressed cytoplasmic PMCA, which is likely involved in Ca 2+ transport and homeostasis, as well as sAC, an evolutionary conserved acid–base sensing enzyme that regulates epithelial ion transport. Furthermore, CA, VHA, and sAC were also expressed within the capillaries that supply blood to the meshwork area, suggesting additional mechanisms that contribute to otolith calcification. This information improves our knowledge about the cellular mechanisms responsible for endolymph ion regulation and otolith formation, and can help understand responses to environmental stressors such as ocean acidification.