1.A.110. The Channel-forming Otopetrin (OTOP) Family
Otopertrin1 of Danio rerio inhibits P2Y purinoceptors. It may modulate calcium homeostasis and influx of calcium in response to extracellular ATP. It is essential for the formation of otoliths in the inner ear of developing larvae and for the perception of gravity and acceleration (Söllner et al. 2004). Otoliths in bony fishes and otoconia in mammals are composite crystals consisting of calcium carbonate and proteins. These biominerals are part of the gravity and linear acceleration detection system of the inner ear. Mutations in otopetrin 1 result in lack of otoconia in tilted and mergulhador mutant mice. Söllner et al. 2004 showed that a mutation in the otop1 gene in zebrafish is responsible for the complete absence of otoliths. They examined the localization of Starmaker, a secreted protein that is abundant in otoliths in backstroke mutants. The starmaker protein accumulated within cells of the otic epithelium, indicating a possible defect in secretion. Otopetrin-1 and Otopetrin-2 are constituents of otoliths, biological crystals formed by a layer of calcium carbonate crystal that adhere to the ciliary surface of the utricular and saccular receptors in the vestibule of all vertebrates inner ears (Huang and Qian 2022).
The fish inner ear contains three large extracellular biomineral particles, otoliths, which have evolved to transduce the force of gravity into neuronal signals. Mammalian ears contain thousands of small particles called otoconia that serve a similar function. Loss or displacement of these structures can be lethal for fish and is responsible for benign paroxysmal positional vertigo (BPPV) in humans. The distinct morphologies of otoconial particles and otoliths suggest divergent developmental mechanisms. Mutations in Otopetrin 1 result in nonsyndromic otoconial agenesis and a severe balance disorder in mice. Hughes et al. 2004 showed that in zebrafish, Otop1 is essential for otolith formation. Morpholino-mediated knockdown of zebrafish Otop1 led to otolith agenesis without affecting the sensory epithelium or other structures within the inner ear. Otop1 regulates cellular Ca2+ (Hughes et al. 2007). The OTOP family was previously the DUF270 or pfam03189 family. The members of this family have three 'Otopetrin Domains' that are highly conserved between vertebrates, arthropods and nematodes, as well as a constrained predicted loop structure (Hughes et al. 2008).
By comparative transcriptome analysis of mouse taste receptor cells, Tu et al. 2018 identified OTOP1 as a proton-selective ion channel. They found that murine OTOP1 is enriched in acid-detecting taste receptor cells and is required for their zinc-sensitive proton conductance. Two related murine genes, Otop2 and Otop3, and a Drosophila ortholog also encode proton channels. Evolutionary conservation of the gene family and its widespread tissue distribution suggest a broad role for proton channels in physiology and pathophysiology (Tu et al. 2018). Structural motifs for subtype-specific pH-sensitive gating of vertebrate otopetrin proton channels have been analyzed (Teng et al. 2022).
Chen et al. 2019 presented the cryo-EM structure of OTOP3 from Xenopus tropicalis (XtOTOP3) along with functional characterization of the channel. XtOTOP3 forms a homodimer with each subunit containing 12 TMSs that can be divided into two structurally homologous halves, each half assembling as an alpha-helical barrel that could serve as a proton conduction pore. Both pores open from the extracellular half before becoming occluded at a central constriction point consisting of three highly conserved residue pairs - Gln232/585-Asp262/Asn623-Tyr322/666 (the constriction triads). Mutagenesis showed that the constriction triad from the second pore is less amenable to perturbation than that of the first pore, suggesting an unequal contribution between the two pores to proton transport. Several key residues at the interface between the two pores that are functionally important, particularly Asp509, which confers intracellular pH-dependent desensitization to OTOP channels, were also identified (Chen et al. 2019).
Otopetrins (Otop1-Otop3) comprise one of two known eukaryotic proton-selective channel families. Otop1 is required for otoconia formation and a candidate mammalian sour taste receptor. Saotome et al. 2019 reported cryo-EM structures of zebrafish Otop1 and chicken Otop3 in lipid nanodiscs. The structures have a dimeric architecture, with each subunit forming 12 TMSs, divided into structurally similar amino (N) and carboxy (C) domains. Cholesterol-like molecules occupy various sites in Otop1 and Otop3 and occlude a central tunnel. In molecular dynamics simulations, hydrophilic vestibules formed by the N- and C-domains and in the intrasubunit interface between N- and C-domains form conduits for water entry into the membrane core, suggesting three potential proton conduction pathways. By mutagenesis, Saotome et al. 2019 tested the roles of charged residues in each putative permeation pathway. The results provided a structural basis for understanding selective proton permeation and gating of this conserved family of proton channels.