Functional Role of Class III Myosins in Hair Cells

Abstract

Cytoskeletal motors produce force and motion using the energy from ATP hydrolysis and function in a variety of mechanical roles in cells including muscle contraction, cargo transport, and cell division. Actin-based myosin motors have been shown to play crucial roles in the development and function of the stereocilia of auditory and vestibular inner ear hair cells. Hair cells can contain hundreds of stereocilia, which rely on myosin motors to elongate, organize, and stabilize their structure. Mutations in many stereocilia-associated myosins have been shown to cause hearing loss in both humans and animal models suggesting that each myosin isoform has a specific function in these unique parallel actin bundle-based protrusions. Here we review what is known about the classes of myosins that function in the stereocilia, with a special focus on class III myosins that harbor point mutations associated with delayed onset hearing loss. Much has been learned about the role of the two class III myosin isoforms, MYO3A and MYO3B, in maintaining the precise stereocilia lengths required for normal hearing. We propose a model for how class III myosins play a key role in regulating stereocilia lengths and demonstrate how their motor and regulatory properties are particularly well suited for this function. We conclude that ongoing studies on class III myosins and other stereocilia-associated myosins are extremely important and may lead to novel therapeutic strategies for the treatment of hearing loss due to stereocilia degeneration.

Keywords: ATPase; MYO3A; MYO3B; actin; force generation; myosin; stereocilia.

FIGURE 1

Representative model of myosin motor localization in inner ear hair cell stereocilia. (A) Three rows of stereocilia form a staircase-like pattern, tethered together by various tip links, that is maintained through an organism’s lifetime. In an individual stereocilium, there are three main regions where myosins localize: (B) the stereocilia tips, also known as the lower tip link density, containing class III myosins and class XV myosins, as well as MET channels, (C) the lower tip link density, containing class I and class VII myosins, and (D) the anklet, containing class VI myosins. MYO7A has been shown to localize throughout the entire length of the stereocilia, however since its proposed function as a tensioning myosin occurs at the UTLD it is shown only here for simplicity. Apart from class III myosins, all stereocilia myosins have direct evidence of membrane binding. However, class III myosins are hypothesized to interact with the membrane, potentially modulated via its binding partner MORN4. Lastly, both MYO7A and MYO15A have not been directly shown to form dimers on their own, however MYO7A can form dimers via a cargo-mediated mechanism and MYO15A was proposed to oligmerize in a complex with cargo. For simplicity, both are represented as dimers.

FIGURE 2

Domain structure of stereocilia-associated myosins. In general, all myosins contain a conserved N-terminal motor domain (head), light chain binding region (neck), and a variable C-terminal domain (tail) which allows specialized functions (e.g., dimerization, filament formation, membrane binding, and cargo/adapter protein binding). Additionally, MYO1C has a membrane binding Pleckstrin-Homology domain, as well as a membrane binding tail homology domain. MYO3A/MYO3B, MYO6, and MYO15A all contain an N-terminal extension domain, with the MYO3A/MYO3B extension being a protein kinase. MYO6 and MYO7A have dimerizing coiled-coil domains, and MYO7A and MYO15A each contain two membrane binding MyTH4/FERM domains.

FIGURE 3

Diagram of the actomyosin ATPase cycle. Myosin cycles between “weak” actin binding and “strong” actin binding states and while strongly bound generates a power stroke that drives filament sliding or walking along actin. Specifically, ATP binding to myosin dissociates it from actin which is followed by ATP hydrolysis and a transition into the pre-power stroke state (recovery stroke). Myosin binding to actin with the hydrolyzed products in its active site leads to strong actin binding, the myosin power stroke, and the release of phosphate. After the power stroke, ADP is released from the nucleotide pocket in a strong actin binding state, which allows for another ATP molecule to bind and start a new ATPase cycle. Abbreviations for actin, myosin, ATP, ADP, ADP.Pi are A, M, T, D, D.Pi, respectively.

FIGURE 4

Working model for MYO3A function in inner ear hair cell stereocilia. MYO3A walks toward the plus end of actin protrusions utilizing its motor and actin binding tail domains and can dock at the tips by an unknown mechanism. The tip localized MYO3A can produce an ensemble force that works together with the actin polymerization forces to combat membrane tension, which can result in protrusion elongation or retraction (depending on the balance of forces). The amount of tip-directed force produced by MYO3A motors at the protrusion tips is impacted by the number of motors present, as well as their duty ratio and intrinsic force producing abilities. Examining how MYO3A motor-based forces are altered by physiological factors (e.g., phosphorylation and protein-protein interactions) or disease associated mutations (e.g., deafness mutations) will be the focus of future investigations that will further test this model.